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hormones

hormones Despite the advent of e-mail the majority of people still communicate by letter or telephone. Similarly there are two ways in which messages are sent round the body. The first is via the nervous system, which like the phone system is ‘hard wired’ and usually operates on a point to point basis. The second way is by means of hormones — chemical messengers — circulating in the blood, which effectively acts as a postal system. Just as, when someone sends out a circular in the mail, those who are interested act on the information and those who are not discard the letter, so when an endocrine gland secretes a hormone the appropriate cells respond while the rest are unaffected. Classically, hormones are defined as chemical substances secreted directly into the bloodstream that act on a distant target organ or type of cell.

Historical background

Diseases resulting from lack of a hormone (such as diabetes mellitus), or excess production (such as thyrotoxicosis), have been known for centuries, although the cause was not recognized. A nineteenth-century anatomist called Henle, after whom a section of the renal tubules was named, was the first person to describe glands without ducts that secreted their products directly into the bloodstream. Then in 1855 the Frenchman, Claude Bernard, who laid the foundations of physiology, distinguished the products of these so-called ductless glands from those glandular secretions, such as saliva and sweat, which are effectively outside the body, by calling them ‘internal secretions’: hence the name ‘endocrine’ (endon: Greek for within) as opposed to ‘exocrine’ secretion (ex: Greek for outside).

The first person who tried to use extracts of endocrine glands for therapeutic purposes was Brown–Sequard, a French physician, neurologist, and endocrinologist, who in 1889 employed testicular extracts from animals to treat male ageing. A few years later, in 1902, Bayliss and Starling, working in University College London, prepared an extract from the duodenum which stimulated secretion of pancreatic digestive juices when it was injected into the bloodstream. They called the product ‘secretin’, and coined the term ‘hormone’, meaning ‘to excite’ or ‘to set in motion’. Since then a wide variety of hormones have been identified. The steps in identifying whether a given gland or tissue has an endocrine function are first to demonstrate changes on its removal and then to demonstrate reversal of those changes, either when the gland is reimplanted at any site where it can link up with a blood supply, or when an extract of the gland is injected into the blood. The active principle can then be isolated, purified, and the chemical structure characterized. Ways of measuring the identified hormone (assays) can be established, and finally one can confirm that venous blood leaving the gland has a higher concentration of the hormone than the arterial blood entering it.

The role of hormones

The major endocrine glands are the pituitary, the thyroid, the four parathyroids, the pancreas, the two adrenals, and the paired testes or ovaries (See endocrine). Hormones are also produced by organs or tissues whose function is not primarily an endocrine one: the digestive tract, the heart, and the kidneys all produce hormones. Even nerve cells produce them. For example, the hormones controlling secretion from the anterior lobe of the pituitary gland are synthesized in the hypothalamus, but they are released into the local blood supply to the anterior pituitary, rather than entering the general circulation. These cells are said to have a neuroendocrine function. Furthermore, it is now recognized that hormones need not even be released into blood vessels. The hormonal products of some nerve cells stimulate adjacent neurones and thus act as neuromodulators, while in the digestive tract hormones act on surrounding cells and are said to have a paracrine function (para: Greek for beside). Finally, some hormones, such as growth factors, can act on the originating cell itself; in this case they are described as exhibiting autocrine control. The classical definition has therefore been extended to include chemical messengers which are secreted by certain cells, and which reach and act upon cells which are receptive to them, whether local or distant.

Chemical nature of hormones

Chemically, most hormones belong to one of three major groups: proteins and peptides, steroids (fat-soluble molecules whose basic structure is a skeleton of four carbon rings), or derivatives of the amino acid tyrosine, characterized by a 6-carbon, or benzene, ring. There are some hormones, such as melatonin from the pineal gland and the locally acting prostaglandins, which cannot be included in any of these groups, but may share a number of their characteristics. The glands which produce protein and peptide hormones are the pituitary, certain cells of the thyroid, the parathyroids, and the pancreas. Steroids are produced by the cortex or outer layer of the adrenal gland and by the ovaries and testes. The tyrosine derivatives are the thyroid hormones, and the catecholamines (adrenaline and noradrenaline) which are produced in the medulla of the adrenal glands.

Knowledge of the chemical nature of a hormone is important as it enables one to predict how the hormone is produced, how rapidly it can be released in response to a stimulus, in what form it circulates in the blood, how it acts, the time course of its effect, and the route of administration therapeutically.

Hormone synthesis and secretion

The mechanisms underlying the synthesis of protein and peptide hormones, such as growth hormone and insulin, are just the same as the synthesis of any other protein, involving transcription of the gene and translation of a messenger RNA (mRNA). Generally the mRNA contains the code for a longer peptide than the normal form of the hormone. These extended forms are called pro-hormones and there may even be pre-pro-hormones, as for example pre-pro-insulin. The active hormone is cleaved from these molecules. The pro-hormone is stored in secretory granules, then released by a process of exocytosis, — the membrane of the storage granule fuses with the plasma membrane, which in turn parts, allowing the contents of the granule to be discharged.

Steroid hormones, such as cortisol and the sex hormones, are all synthesized from cholesterol, with a variety of enzymes mediating the transformations into the different products. Since they are fat soluble, and therefore readily cross membranes, they cannot be stored, but are synthesized as needed. Their release is therefore slower than that of peptide hormones.

The thyroid hormones are formed as part of a large protein, thyroglobulin, which can be stored, while the catecholamines are synthesized by a multi-enzyme process and are also stored in granules.

Neither the steroid hormones nor the thyroid hormones are readily soluble in water, and they circulate in the plasma in association with proteins. The importance of this is that the compound molecules are too large to be filtered out of the blood in the kidney and so are not lost in the urine, which is one of the reasons why they remain in the plasma for days. Peptide hormones, by contrast, disappear within an hour or so, because they are both broken down in plasma and tissues and also lost in the urine. Protein and peptide hormones have therefore to be administered more frequently if used therapeutically, although longer acting preparations are available. Another problem with the administration of these hormones is the fact that they cannot be given by mouth as they would be broken down in the digestive tract. This presents particular problems for diabetics, who have regularly to inject themselves, whereas people with thyroid hormone deficiency only have to take pills.

Hormone action

The chemical nature of the hormone also affects the mechanism of action. All hormones act on cells by way of their ‘receptors’. Each hormone has its own receptor to which it binds, matching rather like a lock and key. This is why hormones circulating throughout the body in the blood may leave capillaries to enter the extracellular fluid of many tissues, but act only on those cells which possess the appropriate receptor. Proteins and peptides cannot enter the cell and so act via cell membrane receptors, producing their effects by ‘second messengers’, which are activated in the cell as soon as the hormone binds to the receptor. Thus peptide hormones can produce quite rapid responses. Steroid and thyroid hormones, by contrast, can enter the cell and bind to intracellular receptors, producing their effects by stimulating the production of new proteins. There is therefore a relatively long lag period before the response to these hormones is seen.

Hormones produce a variety of responses throughout the body and may be grouped according to their actions, although there is overlap between the groups.

First there are the metabolic hormones which control the digestion of food, its storage and use. Such hormones include those produced by the digestive tract, which control secretion of digestive juices and activity of the muscle in the wall of the tract; also the hormones which regulate blood glucose, namely insulin, (which lowers it), and glucagon, growth hormone, the thyroid hormones, and cortisol, which all raise it.

Second are the hormones which regulate the composition of the blood, and hence of all the body fluids. Excluding those that regulate the glucose content, these are: aldosterone and atrial natriuretic hormone (produced in the heart), which control the amount of sodium in the blood; vasopressin or antidiuretic hormone, which controls the amount of water; parathyroid hormone and vitamin D, which raise blood calcium; and calcitonin, which lowers blood calcium. It is perhaps surprising to learn that a vitamin can also be a hormone, but it is similar in many ways to the steroid hormones, and the active form is produced in one part of the body for action an another. The vitamin D taken in the diet or formed in the skin under the action of UV light is not the active form: this is produced after modification takes place first in the liver and then the kidney.

Next are the stress hormones, primarily adrenaline and noradrenaline, which are under the control of the autonomic nervous system: cortisol and a number of the pituitary hormones are also involved in the response to stress.

A further group are those responsible for growth, development, and reproduction. These include growth hormone itself, and the hormones controlling ovarian and testicular function (luteinizing hormone, LH, and follicular stimulating hormone, FSH) — all of which come from the pituitary — and the hypothalamic hormones, which in turn control these pituitary secretions. Included also are the steroid hormones, produced by the ovaries (oestrogens and progesterone) and testes (testosterone), and those hormones involved in birth and lactation, chiefly oxytocin and prolactin.

The final major group includes those hormones that control other endocrine systems, and therefore interact with the other groups. The pituitary hormones adrenocorticotrophic hormone (ACTH), thyroid stimulating hormone (TSH), and the gonadotrophic hormones LH and FSH control the release of some of the metabolic and stress hormones and of the reproductive hormones, whilst hypothalamic hormones in turn control pituitary function.

Regulation of hormone release

The commonest form of control in biological systems is negative feedback, and this forms the basis for the control of hormone release. In this type of feedback loop any perturbation of the controlled variable results in a response to return it to the pre-determined level. An example of this is the control of blood sugar concentrations. A rise in blood glucose (after a sugary drink or food) acts on the pancreas to stimulate insulin secretion, which in turn lowers blood glucose by storing it away inside cells.

A more complex system is seen in the control of pituitary hormone secretion. For hormones which control secretion from a target gland, there is simple negative feedback, with the target organ secretion inhibiting pituitary hormone release (for example, the secretion of thyroid-stimulating hormone is inhibited by a rise in circulating thyroid hormone). However there is also control from the hypothalamus via stimulating and inhibiting hormones. The hypothalamus receives a huge array of inputs originating both in the body and in the external environment, so that by this route a large variety of factors influence the output of the pituitary gland, and hence the other endocrine glands, which it in turn controls.

Endocrine disorders

In a such a complex regulatory system, one would predict that disordered function would have significant consequences. The most common endocrine disorder is diabetes mellitus, with disorders of thyroid function coming second. Endocrine disorders may stem from over- or undersecretion of a given hormone. Oversecretion may be due to a tumour either in the tissue normally producing the hormone or in one growing in an abnormal location — for example in the lung. It may alternatively be due to inappropriate secretion from the whole gland. There is, for example, an autoimmune disease of the thyroid: thyrotoxicosis or ‘Grave's disease’, in which antibodies stimulate the gland to oversecretion. Apparent underactivity of an endocrine gland may in fact be due to a failure of the target tissues to respond to a particular hormone. For example, those who develop diabetes later in life may have an elevated rather than a low concentration of insulin in the blood. This is because their tissues are relatively unresponsive to the hormone. There may even be failure to convert a hormone to its more active form. In the male some tissues are responsive to dihydrotestosterone rather than testosterone itself, and so a deficiency of the enzyme catalyzing this conversion produces the appearance of testosterone deficiency.

Most endocrine disorders can now be successfully treated. Diagnosis and treatment, however, require accurate measurement of blood hormone concentrations. Early assays were bioassays performed on animal tissue, and these are still used in checking the activity of hormone preparations made for medicinal purposes. However, routine determination in blood now involves the technique of radioimmunoassay; when care is taken in setting this up, even very low concentrations of hormone can be determined quite rapidly on a large number of samples.

So the days are past when diabetes mellitus led inexorably to coma and death; when a mother might decline with a mysterious illness after giving birth because of post-partum pituitary degeneration; or when a young woman could ‘burn out’ with thyrotoxicosis — to name but a few of the endocrine disorders which could be seriously debilitating or fatal before the twentieth century.

Mary L. Forsling

Bibliography

Rubenstein, E. (1980). Diseases caused by impaired communication among cells, Scientific American, March, 78–87.
Snyder, S. H. (1985). The molecular basis of communication between cells, Scientific American, October, 114–23.


See endocrine.See also adrenal gland; glands; hypothalamus; insulin; peptides; pituitary gland; sex hormones; steroids; thyroid gland; water balance.

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Hormones

Hormones

Biochemical agents that transmit messages between components of living organisms.

Hormones are biochemical messengers that regulate physiological events in living organisms. More than 100 hormones have been identified in humans. Hormones are secreted by endocrine (ductless) glands such as the hypothalamus , the pituitary gland, the pineal gland, the thyroid, the parathyroid, the thymus, the adrenals, the pancreas, the ovaries, and the testes. Hormones are secreted directly into the blood stream, where they travel to target tissues and modulate digestion, growth, maturation, reproduction, and homeostasis. Hormones do not fall into any one chemical category, but most are either protein molecules or steroid molecules. These biological managers keep the body systems functioning over the long term and help maintain health. The study of hormones is called endocrinology.

Hypothalamus

Most hormones are released into the bloodstream by a single gland. Testosterone is an exception, because it is secreted by both the adrenal glands and by the testes. The major site that keeps track of hormone levels is the hypothalamus. A number of hormones are secreted by the hypothalamus, and they stimulate or inhibit the secretion of hormones at other sites. When the hypothalamus detects high levels of a hormone, it reacts to inhibit further production. When low levels of a hormone are detected, the hypothalamus reacts to stimulate hormone production or secretion. The body handles the hormone estrogen differently. Each month, the Graafian follicle in the ovary releases increasing amounts of estrogen into the bloodstream as the egg develops. When estrogen levels rise to a certain point, the pituitary gland secretes luteinizing hormone (LH), which triggers the egg's release into the oviduct.

The major hormones secreted by the hypothalamus are corticotropin releasing hormone (CRH), thyrotropin releasing hormone (TRH), follicle stimulating hormone releasing hormone (FSHRH), luteinizing hormone releasing hormone (LHRH), and growth hormone releasing hormone (GHRH). CRH targets the adrenal glands. It triggers the adrenals to release adrenocorticotropic hormone (ACTH). ACTH functions to synthesize and release corticosteroids. TRH targets the thyroid where it functions to synthesize and release the thyroid hormones T3 and T4. FSH targets the ovaries and the testes where it enables the maturation of the ovum and of spermatozoa. LHRH also targets the ovaries and the testes, helping to promote ovulation and increase progesterone synthesis and release. GHRH targets the anterior pituitary to release growth hormone to most body tissues, increase protein synthesis, and increase blood glucose.

The hypothalamus also secretes other important hormones such as prolactin inhibiting hormone (PIH), prolactin releasing hormone (PRH), and melanocyte inhibiting hormone (MIH). PIH targets the anterior pituitary to inhibit milk production at the mammary gland, and PRH has the opposite effect. MIH targets skin pigment cells (melanocytes) to regulate pigmentation.

Pituitary gland

The pituitary has long been called the master gland because of the vast extent of its activity. It lies deep in the brain just behind the nose, and is divided into anterior and posterior regions. Both anti-diuretic hormone (ADH) and oxytocin are synthesized in the hypothalamus before moving to the posterior pituitary prior to secretion. ADH targets the collecting tubules of the kidneys, increasing their permeability to and retention of water. Lack of ADH leads to a condition called diabetes insipidus characterized by excessive urination. Oxytocin targets the uterus and the mammary glands in the breasts. Oxytocin also triggers labor contractions prior to birth and functions in the ejection of milk. The drug pitocin is a synthetic form of oxytocin and is used medically to induce labor.

The anterior pituitary (AP) secretes a number of hormones, including growth hormone (GH), ACTH, TSH, prolactin, LH, and FSH. GH controls cellular growth, protein synthesis, and elevation of blood glucose concentration. ACTH controls secretion of some hormones by the adrenal cortex (mainly cortisol). TSH controls thyroid hormone secretion in the thyroid. In males, prolactin enhances testosterone production; in females, it initiates and maintains LH to promote milk secretion from the mammary glands. In females, FSH initiates ova development and induces ovarian estrogen secretion. In males, FSH stimulates sperm production in the testes. LH stimulates ovulation and formation of the corpus luteum, which produces progesteronein females, whereas LH stimulates interstitial cells in males to produce testosterone.

Thyroid gland

The thyroid lies under the larynx and synthesizes two hormones, thyroxine and tri-iodothyronine. This gland takes up iodine from the blood and has the highest iodine level in the body. The iodine is incorporated into the thyroid hormones. Thyroxine has four iodine atoms and is called T4. Tri-iodothyronine has three iodine atoms and is called T3. Both T3 and T4 function to increase the metabolic rate of several cells and tissues. The brain, testes, lungs, and spleen are not affected by thyroid hormones, however. T3 and T4 indirectly increase blood glucose levels as well as the insulin-promoted uptake of glucose by fat cells. Their release is modulated by TRH-RH from the hypothalamus. When temperature drops, a metabolic increase is triggered by TSH. Chronic stress seems to reduce TSH secretion which, in turn, decreases T3 and T4 output.

Depressed T3 and T4 production is the trademark of hypothyroidism. If it occurs in young children, this decreased activity can cause physical and mental retardation . In adults, it creates sluggishnessmentally and physicallyand is characterized further by weight gain, poor hair growth, and a swollen neck. Excessive T3 and T4 cause sweating, nervousness, weight loss, and fatigue. The thyroid also secretes calcitonin, which serves to reduce blood calcium levels. Calcitonin's role is particularly significant in children whose bones are still forming.

Parathyroid glands

The parathyroid glands are attached to the bottom of the thyroid gland. They secrete the polypeptide parathyroid hormone (PTH), which plays a crucial role in monitoring blood calcium and phosphate levels. Calcium is a critical element for the human body. Even though the majority of calcium is in bone, it is also used by muscles, including cardiac muscle, for contractions, and by nerves in the release of neurotransmitters. Calcium is a powerful messenger in the immune response of inflammation and blood clotting. Both PTH and calcitonin regulate calcium levels in the kidneys, the gut, bone, and blood.

PTH deficiency can be due to autoimmune diseases or to inherited parathyroid gland problems. Low PTH capabilities cause depressed blood calcium levels and neuromuscular problems. Very low PTH can lead to tetany or muscle spasms. Excess PTH can lead to weakened bones because it causes too much calcium to be drawn from the bones and to be excreted in the urine. Abnormalities of bone mineral deposits can lead to a number of conditions, including osteoporosis and rickets. Osteoporosis can be due to dietary insufficiencies of calcium, phosphate, or vitamin C. The end result is a loss of bone mass. Rickets is usually caused by a vitamin D deficiency and results in lower rates of bone formation in children. These examples show the importance of a balanced, nutritious diet for healthy development.

Adrenal glands

The two adrenal glands sit one on top of each kidney. Both adrenals have two distinct regions. The outer region (the medulla) produces adrenaline and noradrenaline and is under the control of the sympathetic nervous system . The inner region (the cortex) produces a number of steroid hormones. The cortical steroid hormones are derived from cholesterol and include mineralocorticoids (mainly aldosterone), glucocorticoids (mainly cortisol), and gonadocorticoids. Aldosterone and cortisol are the major human steroids in the cortex. However, testosterone and estrogen are secreted by adults (both male and female) at very low levels.

Aldosterone plays an important role in regulating body fluids. It increases blood levels of sodium and water and lowers blood potassium levels. Cortisol secretion is stimulated by physical trauma, exposure to cold temperatures, burns, heavy exercise, and anxiety. Cortisol targets the liver, skeletal muscle, and adipose tissue, and its overall effect is to provide amino acids and glucose to meet synthesis and energy requirements for metabolism and during periods of stress. Because of its anti-inflammatory action, cortisol is used clinically to reduce swelling. Excessive cortisol secretion leads to Cushing's syndrome, which is characterized by weak bones, obesity , and a tendency to bruise. Cortisol deficiency can lead to Addison's disease, which has the symptoms of fatigue, low blood sodium levels, low blood pressure, and excess skin pigmentation.

The adrenal medullary hormones are epinephrine (adrenaline) and nor-epinephrine (nor-adrenaline). Both of these hormones serve to supplement and prolong the "fight or flight" response initiated in the nervous system. This response includes increased heart rate, peripheral blood vessel constriction, sweating, spleen contraction, glycogen conversion to glucose, dilation of bronchial tubes, decreased digestive activity, and low urine output.

Pancreas

The pancreas secretes the hormones insulin, glucagon, and somatostatin, also known as growth hormone inhibiting hormone (GHIH). Insulin and glucagon have reciprocal roles. Insulin promotes the storage of glucose, fatty acids, and amino acids, while glucagon stimulates mobilization of these constituents from storage into the blood. Insulin release is triggered by high blood glucose levels. It lowers blood sugar levels and inhibits the release of glucose by the liver in order to keep blood levels down. Insulin excess can cause hypoglycemia leading to convulsions or coma , and insufficient levels of insulin can cause diabetes mellitus, which can be fatal if left untreated. Diabetes mellitus is the most common endocrine disorder.

Glucagon secretion is stimulated by decreased blood glucose levels, infection, cortisol, exercise, and large protein meals. Among other activities, it facilitates glucose release into the blood. Excess glucagon can result from tumors of the pancreatic alpha cells, and a mild diabetes seems to result. Some cases of uncontrolled diabetes are also characterized by high glucagon levels, suggesting that low blood insulin levels are not necessarily the only cause in diabetes cases.

Female hormones

The female reproductive hormones arise from the hypothalamus, the anterior pituitary, and the ovaries. Although detectable amounts of the steroid hormone estrogen are present during fetal development, at puberty estrogen levels rise to initiate secondary sexual characteristics. Gonadotropin releasing hormone (GRH) is released by the hypothalamus to stimulate pituitary release of LH and FSH, which propagate egg development in the ovaries. Eggs (ova) exist at various stages of development, with the maturation of one ovum taking about 28 days. The ova are contained within follicles that are support organs for ova maturation. About 450 of a female's 150,000 germ cells mature to leave the ovary. The hormones secreted by the ovary include estrogen, progesterone, and small amounts of testosterone.

As an ovum matures, rising estrogen levels stimulate additional LH and FSH release from the pituitary. Prior to ovulation, estrogen levels drop, and LH and FSH surge to cause the ovum to be released into the fallopian tube. The cells of the burst follicle begin to secrete progesterone and some estrogen. These hormones trigger thickening of the uterine lining, the endometrium, to prepare it for implantation should fertilization occur. The high progesterone and estrogen levels prevent LH and FSH from further secretionthus hindering another ovum from developing. If fertilization does not occur, eight days after ovulation the endometrium deteriorates, resulting in menstruation. The falling estrogen and progesterone levels that follow trigger LH and FSH, starting the cycle all over again.

In addition to its major roles in the menstrual cycle, estrogen has a protective effect on bone loss, which can lead to osteoporosis.

Hormones related to pregnancy include human chorionic gonadotrophin (HCG), estrogen, human chorionic somatomammotrophin (HCS), and relaxin. HCG is released by the early embryo to signal implantation. Estrogen and HCS are secreted by the placenta. As birth nears, relaxin is secreted by the ovaries to relax the pelvic area in preparation for labor.

Male hormones

Male reproductive hormones come from the hypothalamus, the anterior pituitary, and the testes. As in females, GRH is released from the hypothalamus, which stimulates LH and FSH release from the pituitary. Testosterone levels are quite low until puberty. At puberty, rising levels of testosterone stimulate male reproductive development including secondary characteristics. LH stimulates testosterone release from the testes. FSH promotes early spermatogenesis. The male also secretes prostaglandins. These substances promote uterine contractions which help propel sperm towards an egg during sexual intercourse. Prostaglandins are produced in the seminal vesicles, and are not classified as hormones by all authorities.

Further Reading

Little, M. The Endocrine System. New York: Chelsea House Publishers, 1990.

Parker, M., ed. Steroid Hormone Action. New York: IRL Press, 1993.

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Hormones

Hormones

A hormone is a chemical that is produced in one tissue and transported via the circulatory system to a different target tissue. There, it causes a physiological change in the target.

Hormones are the chemical messengers of the endocrine system . The endocrine system also includes the ductless glands that synthesize and secrete hormones, and incorporates the responding target cells as well. Hormones are secreted by endocrine glands directly into the circulatory system, from which they contact nearly all cells of the body. Some endocrine glands, such as the adrenal glands, form organs of their own, while others are just parts of organs. The brain, for example, performs certain critical endocrine functions.

The endocrine system is one of two physiological systems responsible for the control of all biological processes. The other is the nervous system. While the nervous system controls specific, rapid biological responses, often to external stimuli, endocrine control generally involves comparatively broad, long-term, gradual physiological processes.

The endocrine system is essential to diverse aspects of an organism's biology, including its development, growth, reproduction, metabolism, water and ionic balance, and maintenance of homeostasis (internal equilibrium ). In general, animal species that are characterized by well-developed nervous and circulatory systems also possess endocrine control systems.

Because hormones are transported through the circulatory system, they come into contact with all cells and are able to affect numerous tissues simultaneously. Some hormones affect a wide variety of tissues. The sex hormone testosterone, for example, affects multiple parts of the body, whereas others have a considerably more limited effect.

Only cells that possess receptors specific to a hormone will respond to its presence. In addition, depending on the hormone receptor and the pathway coupled to it, different tissues can respond to the same hormone in different ways. Thus, despite their relatively low concentrations in the bloodstream, hormones can have dramatic effects on an organism's physiology.

The Two Major Hormone Groups

Hormones have been divided into two major groups that differ in their biochemical attributes, as well as in the mechanisms by which they affect the activity of target cells. These are steroid hormones and peptide hormones.

Steroid hormones are synthesized by endocrine glands in the gonads (ovaries and testes) and adrenal cortex. They are not stored but, rather, secreted into the circulatory system as soon as they are synthesized.

Steroid hormones are derived from cholesterol and are lipid soluble. Lipid solubility enables steroid hormones to cross cell membranes and enter directly into the cytoplasm . Once there, hormone molecules bind to cytoplasmic receptors, cross the nuclear membrane, and interact directly with DNA to affect cellular activity. Some well-known steroids are estrogen and testosterone.

Peptide hormones, on the other hand, are proteins and composed of amino acids. Peptide hormones are water soluble and range greatly in size. They are synthesized in endocrine cells and then stored in vesicles within the cell for secretion later.

Peptides are the more diverse group of hormones by far. Unlike steroids, peptide hormones are not lipid soluble and do not penetrate their target cells directly. Instead, they function via what is referred to as a second messenger pathway. The hormone binds to a receptor protein on the target cell membrane, which then signals a second messenger within the cellular cytoplasm. This second messenger initiates an enzyme cascade, which affects the activity of the cell. Examples of second messengers involved in peptide hormone function include cyclic AMP and inositol triphosphate.

Endocrine Control

The maintenance of appropriate hormone concentrations in the bloodstream is absolutely critical. Numerous diseases result from hormone levels that are too high or too low. Diabetes is one well-known example.

Feedback systems are often used to regulate hormone synthesis and secretion. Some of these cycles can be extremely complex, involving numerous hormones and endocrine glands.

A particularly well-studied example is the control of thyroid hormone levels. The hypothalamus, an endocrine organ in the brain, secretes a hormone called the thyroid-releasing hormone (TRH). TRH targets the anterior pituitary , which responds by secreting thyroid-stimulating hormone (TSH).

TSH targets the thyroid, inducing it to secrete the thyroid hormones known as T3 and T4. However, when T3 and T4 reach a certain concentration in the bloodstream, they act on the hypothalamus, inhibiting it from secreting more TRH. As a result, TSH is no longer secreted, and T3 and T4 secretion is also terminated. This type of negative feedback is common in endocrine regulation. When the levels of thyroid hormones fall below a certain concentration in the bloodstream, the inhibitory, or restraining, effect on the hypothalamus is removed.

The hypothalamus and the anterior pituitary (which is often referred to as the master gland) are critical to endocrine control because many of the hormones they produce affect the activity of other endocrine glands. The hypothalamus is located at the base of the middle portion of the brain, and the pituitary lies immediately below it. The two are directly connected by blood vessels, an unusual organization of the circulatory system referred to as a portal system. The portal system allows for the direct and efficient transport of hormones from the hypothalamus to targets within the pituitary.

Other hormones are under cyclical control. Cycles can be short, lasting hours, or much longer, spanning several months. Melatonin is a hormone produced by the pineal gland whose level follows a daily cycle. It establishes circadian rhythms . Hormone cycling over longer periods is responsible for the control of activities such as menstruation, hibernation, and seasonal mating behavior.

Important Endocrine Glands and Hormones

One major endocrine gland is the anterior pituitary. It secretes growth hormone as well as gonadotropins, which stimulate sex hormone production in the gonads, and prolactin, which is associated with lactation. Another important endocrine gland is the posterior pituitary. It secretes antidiuretic hormone, one of the key players involved in water balance, and oxytocin, which induces uterine contractions during childbirth.

Other significant endocrine glands can be cited. The thyroid is responsible for the thyroid hormones T3 and T4, which regulate growth, development, and metabolism. Of the adrenal glands, the adrenal medulla produces epinephrine and norepinephrine, while the adrenal cortex produces steroid hormones including the mineralocorticoids and glucocorticoids. The pancreas secretes insulin and glucagon, two antagonistic hormones that together regulate blood glucose levels. Finally, there are the thymus, the pineal gland, and the ovaries and testes, which produce sex hormones.

see also Behavior; Dominance Hierarchy.

Jennifer Yeh

Bibliography

Curtis, Helena. Biology. New York: Worth Publishers, 1989.

Gould, James L., and William T. Keeton. Biological Science, 6th ed. New York: W.W. Norton, 1996.

Hickman, Cleveland P., Larry S. Roberts, and Allan Larson. Animal Diversity. Dubuque, IA: William C. Brown, 1994.

Hildebrand, Milton, and Viola Hildebrand. Analysis of Vertebrate Structure. New York:John Wiley, 1994.

Withers, Philip C. Comparative Animal Physiology. Fort Worth, TX: Saunders College Publishing, 1992.

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Hormone

Hormone

Hormones are chemical messengers that regulate bodily processes such as growth, reproduction, metabolism, digestion, mineral and fluid balance, and the functioning of various organs. In animals, hormones are secreted by organs, tissues, and glands of the endocrine system directly into the blood by and carried in the bloodstream to target organs. Once there, they alter the activities of the organ or regulate the production of other hormones.

Hormones aid in determining an animal behavior patterns and also the probability that a particular behavior will occur. Hormones exert substantial control over the following behavior patterns: parental care, territorial behavior, metamorphosis (in insects), foraging behavior, and circadian rhythms (behavior patterns that always occur at the same time each day). Most hormones fall into two main categories: peptides (chains of amino acids) and lipids (which include steroids).

The Endocrine System

The endocrine system produces many hormones. The major endocrine glands are the pituitary, located at the base of the brain, the thyroid and the parathyroid in the neck, and the pancreas, adrenals, and gonads (reproductive glands) in the torso. Hormones are also produced by the stomach, the small intestine, and the kidneys.

The Pituitary Gland

The tiny pituitary gland was once considered to be the "master gland" of the body. Today, scientists realize that the hypothalamus modulates the activities of the pituitary. The pituitary gland is composed of two lobes, the anterior pituitary and the posterior pituitary. The anterior pituitary produces six major hormones, and the posterior pituitary stores two hormones originating in the hypothalamus. The pituitary's target endocrine glands are the thyroid, adrenal gland, and the gonads. Through these glands it controls the growth of the skeleton and regulates the functions of the thyroid and the gonads. One pituitary hormone, called growth hormone, must be secreted in just the right amount for normal growth in childhood. If too little is produced, the child will become a dwarf; if too much is secreted, the child will grow to be a giant.

Thyroid Hormones

Thyroid hormones stimulates oxygen consumption and metabolism, regulating the growth of body tissues and the rate at which food is burned to provide body energy. They also increase the sensitivity of some organs, especially the central nervous system. If the thyroid becomes overactive, it produces a condition called hyperthyroidism, which causes nervousness and irritability. Another thyroid condition, cretinism, is caused by a congenital lack of thyroid secretion. It is marked by greatly stunted physical and mental growth.

Insulin and Glucagon

The pancreas produces two important hormones, insulin and glucagon. Insulin affects most cells in the body because it is involved in the metabolism of carbohydrates, proteins, and fat. Too little insulin results in diabetes, a condition of high levels of blood sugar resulting in weakness and dehydration. Too much insulin causes very low levels of blood sugar, resulting in weakness, anxiety, and convulsions. Glucagon raises the blood sugar level. Together, insulin and glucagon help keep a normal level of glucose in the blood.

Adrenal Glands and Gonads

Hormones in the adrenal glands control the concentration of salts and water in body fluids and are necessary for maintaining life. They also produce sugar from proteins and store it in the liver to help maintain resistance to physical and emotional stress.

Hormones found in the gonads control sexual development and reproductive processes. A fetus's sex is determined by genetics, but certain hormones produced by the gonads (under the influence of the pituitary gland) must be present for the fetus to develop appropriate sex organs.

Early Discoveries

The term hormone (from the Greek for "to spur on") was first used by the British biochemists William Bayliss and Emest Starling in 1904. The duo coined the term to describe the action of a digestive substance they had isolated called secretin, which stimulates the flow of pancreatic juice. Scientists later realized that the first hormone to have actually been isolated and synthesized (artificially created) was the adrenal hormone epinephrine, identified by Japanese American chemist Jokichi Takemine in 1901.

The isolation of the thyroid hormone thyroxine in 1914 by American biochemist Edward Kendall marked another important milestone in understanding how hormones work. Too much or too little thyroxine can cause illness. One of the earliest thyroid disorders diagnosed was Graves' disease (a disease of the thyroid gland resulting in increased size and activity of the gland). Its cause is unknown, but it is believed to be an autoimmune disorder, and it occurs most often in women. Graves' disease often results in bulging eyes, tachycardia (fast and irregular heartbeat), and thickening of the skin.

One of the most well known developments in endocrinology was the isolation of insulin by the Canadian physicians Frederick Banting and Charles Best in 1921. Soon various types of injectable insulin were being used to treat diabetes.

The 1920s also saw the discovery that the pituitary gland stimulates the sex organs and the introduction (in 1928) of the first pregnancy test. Soon after, the relationship between female sex hormones and the menstrual cycle was explained. Working from this relationship, Gregory Pincus would introduce the first oral contraceptives in the 1950s.

In the 1920s and 1930s it was also learned that the adrenal glands contain hormones that control the concentration of salts and water in body fluids and are essential for maintaining life. Adrenal hormones are also essential for sugar and protein formation and storage in the liver. They also help resist physical and emotional stresses In the 1930s, Kendall and the Swiss chemist Tadeus Reichstein both isolated one of these hormones, cortisone, which is a steroid.

American researcher Philip Hench used cortisone to reduce inflammation in rheumatoid arthritis and other connective tissue diseases in the 1940s, making cortisone the first hormone to be used medically.

Synthetic Hormones

Scientists eventually learned to make some hormones in the laboratory. Vincent Du Vigneaud, an American biochemist, synthesized the small pituitary hormone oxytocin, which regulates milk production in the mammary glands and causes uterine contractions. This led to the synthesis of many larger and more complex hormones for medical purposes.

Today, hormone production can be automated, yielding a great deal of synthetic hormone at a rapid rate to meet increasing medical demands. Patients with hormone deficiencies can often be treated effectively with these artificial hormones. Diabetics, for example, receive insulin. Patients suffering from dwarfism are given human growth hormone. Oral contraception combines the use of estrogen and progesterone to prevent ovulation and thus pregnancy. Hormones are also used to treat infertility.

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Hormones

Hormones

Hormones are chemicals produced by one kind of tissue in an organism and then transported to other tissues in the organism, where they produce some kind of response. Because of the way they operate, hormones are sometimes called "chemical messengers." Hormones are very different from each otherdepending on the functions they performand they occur in both plants and animals.

An example of hormone action is the chemical known as vasopressin. Vasopressin is produced in the pituitary gland (at the base of the brain) of animals and then excreted into the bloodstream. The hormone travels to the kidneys, where it causes an increase in water retention. Greater water retention produces, in turn, an increase in blood pressure.

Plant hormones

Some of the earliest research on hormones involved plants. In the 1870s, English naturalist Charles Darwin (18091882) and his son Francis (18481925) studied the effect of light on plant growth. They discovered that plants tend to grow towards a source of light. They called the process phototropism. The reason for this effect was not discovered for another half century. Then, in the 1920s, Dutch-American botanist Frits Went (18631935) discovered the presence of certain compounds that control the growth of plant tips toward light. Went named those compounds auxins. Auxins are formed in the green tips of growing plants, in root tips, and on the shaded side of growing shoots. They alter the rate at which various cells in the plant grow so that it always bends towards the light.

Words to Know

Auxins: A group of plant hormones responsible for patterns of plant growth.

Endocrine glands: Glands that produce and release hormones in an animal.

Phototropism: The tendency of a plant to grow towards a source of light.

Plant growth regulators: Plant hormones that affect the rate at which plants grow.

Many other plant hormones have since been discovered. These hormones are also called plant growth regulators because they affect the rate at which roots, stems, leaves, or other plant parts grow. The gibberellins,

Important Hormones of the Human Body

Hormone Source Function
Adrenalin (epinephrine) Adrenal gland Initiates emergency "fight or flight" responses in the nervous system
Androgens (including testosterone) Testes Develop and maintain sex organs and male secondary sex characteristics
Cortisone and related hormones Adrenal gland Control the metabolism (breaking down) of carbohydrates and proteins (to produce energy), maintain proper balance of electrolytes (which regulate the electric charge and flow of water molecules across cell membranes), and reduce inflammation
Digestive hormones Various parts of the digestive system Make possible various stages of digestion
Estrogen Ovaries and uterus Develops sex organs and secondary female sexual characteristics; maintains pregnancy
Glucagon Pancreas (Islets of Langerhans) Raises blood glucose (sugar) levels
Gonadotropic hormones Pituitary gland Stimulate gonads (sex organs)
Growth hormone Pituitary gland Stimulates growth of skeleton and gain in body weight
Insulin Pancreas (Islets of Langerhans) Lowers blood glucose levels
Oxytocin Pituitary gland Causes contraction of some smooth muscles
Progesterone Ovaries and uterus Influences menstrual cycle and maintains pregnancy
Thyroxine Thyroid gland Regulates rate of metabolism and general growth rate
Vasopressin Pituitary gland Reduces loss of water from kidneys

for example, are chemicals that occur in many different kinds of plants. They cause cells to divide (reproduce) more quickly and to grow larger in size. Another group of plant growth regulators is the cytokinins. One interesting effect of the cytokinins is that they tend to prevent leaves from aging. When placed on a yellow leaf, a drop of cytokinin can cause the leaf to turn green again.

Animal hormones

Hundreds of different hormones have been discovered in animals. The human body alone contains more than 100 different hormones. These hormones are secreted by endocrine glands, also known as ductless glands. Examples of endocrine glands include the hypothalamus, pituitary gland, pineal gland, thyroid, parathyroid, thymus, adrenals, pancreas, ovaries, and testes. Hormones are secreted from these glands directly into the bloodstream. They then travel to target tissues and regulate digestion, growth, maturation, reproduction, and homeostasis (maintaining the body's chemical balance).

[See also Diabetes mellitus; Endocrine system; Reproductive system; Stress ]

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Hormones

Hormones

Hormones are molecules released by a group of cells in the body that influence the behavior of another group of cells. Hormones are the chemical signals of the endocrine system, the group of glands that, along with the nervous system, controls the body's responses to internal and external stimuli. Hormones are carried to their target cells in the bloodstream.

All hormones bind at the target cell to a specific receptor, a protein made by the target cell. When the hormone binds to the receptor, it causes a change in the receptor's conformation , or shape. This conformation change allows the receptor to fit with other cell molecules in a way it could not before, thus triggering new activities in the cell. While a hormone such as testosterone (produced in the testes) reaches all cells in the body, only some cells have testosterone receptors, and therefore only those cells are sensitive to testosterone's effects. Similarly, different receiving cells make different sets of molecules to interact with the testosterone receptor, and this controls the exact response the target cell exhibits.

Hormones are classified based on their chemical structures. Peptide hormones are chains of amino acids . Insulin and glucagon, which help control blood sugar, are peptide hormones, as are the hormones of the hypothalamus and the pituitary gland. Steroid hormones are lipids (fatlike molecules) whose structures are derived from cholesterol. Hormones of the sex organs and the adrenal cortex (part of the adrenal gland) are steroids. Monoamine hormones are made by modifying amino acids. These hormones include adrenaline and noradrenaline made by the adrenal medulla, thyroid hormone (thyroxine), and melatonin from the pineal gland in the brain.

Hormones also differ in where their receptors are found in the target cell, and the type of effect they cause when they bind to their receptors. The receptor for thyroxine is located in the nucleus , while the receptors for steroid hormones are found in the cell's cytoplasm . In both cases, the hormone binds to the receptor to form a complex, and then the hormone-receptor complex activates specific genes within the nucleus, leading to synthesis of new proteins.

Adrenaline, noradrenaline, and the peptide hormones do not enter the target cell. Instead, they bind to a receptor on the membrane surface. The receptor extends through the membrane, and when the outside portion binds to the hormone, the inside portion of the receptor undergoes a conformation change. This change sets off a cascade of reactions inside the cell, ultimately leading to an increase in concentration of one or another internal messenger molecules. The most common of these so-called "second messengers" (the hormone is the "first messenger") are calcium ion and cyclic AMP (cAMP), a type of nucleotide . The second messenger then triggers other activities in the cell, depending on the cell type. In muscle, adrenaline causes cAMP buildup, which causes breakdown of glycogen to release glucose , which the muscle cell uses to support increased activity.

Hormones that bind to external receptors and work through second messengers affect pre-existing proteins within the cell. Because of this, they typically cause much faster effects than those that bind to internal receptors, which influence creation of new proteins. For example, adrenaline's effects last from minutes to hours at the most, while testosterone's effects last from days to months or more.

see also Adrenal Gland; Amino Acid; Blood Sugar Regulation; Endocrine System; Female Reproductive System; Homeostasis; Hypothalamus; Male Reproductive System; Nucleotides; Pancreas; Pituitary Gland; Thyroid Gland; Transcription

Richard Robinson

Bibliography

Alberts, Bruce, et al. Molecular Biology of the Cell, 4th ed. New York: Garland Publishing, 2000.

Saladin, Kenneth S. Anatomy and Physiology: The Unity of Form and Function. Dubuque, IA: McGraw-Hill Higher Education, 2001.

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hormone

hormone, secretory substance carried from one gland or organ of the body via the bloodstream to more or less specific tissues, where it exerts some influence upon the metabolism of the target tissue. Normally, various hormones are produced and secreted by the endocrine glands (see endocrine system), including the pituitary, thyroid, parathyroids, adrenals, ovaries, testes, pancreatic islets, certain portions of the gastrointestinal tract, and the placenta, among the mammalian species. As lack of any one of them may cause serious disorders, many hormones are now produced synthetically and used in treatment where a deficiency exists. The hormones of the anterior pituitary include thyrotropin, adrenocorticotropic hormone, the gonadotropic hormones, and growth hormone; the posterior pituitary secretes antidiuretic hormone, prolactin, and oxytocin. The thyroids secrete thyroxine and calcitonin, and the parathyroids secrete parathyroid hormone. The adrenal medulla secretes epinephrine and norepinephrine while the cortex of the same gland releases aldosterone, corticosterone, cortisol, and cortisone. The ovaries primarily secrete estrogen and progesterone and the testes testosterone. The adrenal cortex, ovaries, and testes in fact produce at least small amounts of all of the steroid hormones. The islets of Langerhans in the pancreas secrete insulin, glucagon, and somatostatin. The kidneys also produce erythropoietin, which produces erythrocytes (red blood cells). The passage of chyme (see digestive system) from the stomach to the duodenum causes the latter to release secretin, which stimulates the flow of pancreatic juice. The duodenum can also be stimulated by the presence of fats in the chyme to secrete cholecystokinin, a hormone that stimulates the gall bladder to contract and release bile. There is evidence that the upper intestine secretes pancreatozymin, which enhances the amount of digestive enzymes in the pancreatic juice. In addition, the pyloric region of the stomach secretes gastrin, a hormone that increases the secretion of hydrochloric acid into the stomach. The placenta has been shown to secrete progesterone and chorionic gonadotropin. There is evidence that it even contains a substance similar to growth hormone. Insects have a unique hormonal system that includes ecdysone, a steroid that influences molting and metamorphosis, and juvenile hormone, needed for early development. Plants, too, have a hormonal system, which includes the auxins, the gibberellins, the cytokinins, and substances associated with the formation of flowers, tubers, bulbs, and buds. Ethylene is said to function as a hormone in plants, acting to hasten the ripening of fruits.

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hormone

hormone Chemical substance secreted by living cells.Hormones affect the metabolic activities of cells in other parts of the body. In mammals, glands of the endocrine system secrete hormones directly into the bloodstream. Hormones exercise chemical control of physiological functions, regulating growth, development, sexual functioning, metabolism, and (in part) emotional balance. They maintain a delicate equilibrium that is vital to health. The hypothalamus, adjacent to the pituitary gland at the base of the brain, is responsible for overall coordination of the secretion of hormones. Most hormones are proteins or steroids. Hormones include thyroxine, adrenaline, insulin, oestrogen, progesterone, and testosterone. In plants, hormones control many aspects of metabolism, including cell elongation and division, direction of growth, initiation of flowering, development of fruits, leaf fall, and responses to environmental factors. The most important plant hormones include auxin, gibberellin, and cytokinin. See also homeostasis

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Hormones

Hormones

Hormones are small molecules that are released by one part of a plant to influence another part. The principal plant growth hormones are the auxins, gibberellins, cytokinins, abscisic acid, and ethylene. Plants use these hormones to cause cells to elongate, divide, become specialized, and separate from each other, and help coordinate the development of the entire plant. Not only are the plant hormones small in molecular weight, they are also active in the plant in very small amounts, a fact that made their isolation and identification difficult.

The first plant growth hormones discovered were the auxins. (The term auxin is derived from a Greek word meaning "to grow.") The best known and most widely distributed hormone in this class is indole-3-acetic acid. Fritz W. Went, whose pioneering and ingenious research in 1928 opened the field of plant hormones, reported that auxins were involved in the control of the growth movements that orient shoots toward the light, and that they had the additional, striking quality of moving only from the shoot tip toward the shoot base. This polarity of auxin movement was an inherent property of the plant tissue, only slightly influenced by gravity. Other less-investigated auxins include phenyl-acetic acid and indole-butyric acid, the latter long used as a synthetic auxin but found to exist in plants only in 1985.

The gibberellins are a family of more than seventy related chemicals, some active as growth hormones and many inactive. They are designated by number (e.g., GA1 and GAL20). GA3 (also called gibberellic acid) is one of the most active gibberellins when added to plants. Slight modifications in the basic structure are associated with an increase, decrease, or cessation of biological activity: each such modified chemical is considered a different gibberellin.

Cytokinins are a class of chemical compounds derived from adenine that cause cells to divide when an auxin is also present. Of the cytokinins found in plants, zeatin is one of the most active.

Abscisic acid helps protect the plant from too much loss of water by closing the small holes (stomata) in the surfaces of leaves when wilting begins.

PLANT HORMONES AND THEIR FUNCTIONS
Hormone Functions
Auxins (indoleacetic acid; IAA) Stimulates shoot and root growth; involved in tropisms; prevents abscission; controls differentiation of xylem cells and, with other hormones, controls sieve-tube cells and fibers
Gibberellins Stimulates stem elongation, seed germination, and enzyme production in seeds
Cytokinins Stimulates bud development; delays senescence; increases cell division
Abscisic acid Speeds abscission; counters leaf wilting by closing stomates; prevents premature germination of seeds; decreases IAA movement
Ethylene (gas) Produced in response to stresses and by many ripening fruits; speeds seed germination and the ripening of fruit, senescence, and abscission; decreases IAA movement

The only known gas that functions as a plant growth hormone is the small C2 H2 molecule called ethylene. Various stresses, such as wounding or waterlogging, lead to ethylene production.

Major Effects of the Principal Plant Growth Hormones

Auxins.

Indoleacetic acid (IAA), produced primarily in seeds and young leaves, moves out of the leaf stalk and down the stem, controlling various aspects of development on the way. IAA stimulates growth both in leaf stalks and in stems. In moving down the leaf stalk, IAA prevents the cells at the base of the leaf from separating from each other and thus causing the leaf to drop (called leaf abscission). The speed of IAA polar movement through shoot tissues ranges from 5 to 20 millimeters per hour, faster than speeds for the other major hormones.

The growth responses of plants to directional stimuli from the environment are called tropisms. Gravitropism (also called geotropism) refers to a growth response toward or away from gravity. Phototropism is the growth response toward or away from light. These tropisms are of obvious value to plants in facilitating the downward growth of roots into the soil (by positive gravitropism) and the upward growth of shoots into the light (by positive phototropism, aided by negative gravitropism).

The role of auxin in controlling tropisms was suggested by Went and N. Cholodny in 1928. Their theory was that auxin moves laterally in the shoot or root under the influence of gravity or one-sided light. Greater concentration on one side causes either greater growth (in the case of the shoot) or inhibited growth (in roots). This Cholodny-Went theory of tropisms has been subject to refinement and question for decades. Evidence exists, for instance, that in some plants tropism toward one-sided light results not from lateral movement of auxin to the shaded side, but rather from production of a growth inhibitor on the illuminated side.

A widespread, though not universal, effect of IAA moving down from the young leaves of the apical bud is the suppression of the outgrowth of the side buds on the stem. This type of developmental control is called apical dominance: if the apical bud is cut off, the side buds start to grow out (released from apical dominance). If IAA is applied to the cut stem, the side buds remain suppressed in many plants.

In addition to enhancing organ growth, IAA also plays a major part in cell differentiation, controlling the formation of xylem cells and being involved in phloem differentiation. In its progress down the stem, IAA stimulates the development of the two main vascular channels for the movement of substances within the plant: xylem, through which water, mineral salts, and other hormones move from the roots; and phloem, through which various organic compounds such as sugars move from the leaves. In plants that develop a cambium (the layer of dividing cells whose activity allows trees to increase in girth), the polarly moving IAA stimulates the division of the cambial cells. Cut-off pieces of stem or root usually initiate new roots near their bases. As a result of its polar movement, IAA accumulates at the base of such excised pieces and touches off such root regeneration. In the intact plant, the shoot-tip toward shoot-base polar movement of IAA continues on into the root, where IAA moves toward the root tip primarily in the stele (the inner column of cells in the root).

Interesting effects of IAA have been found in a more limited number of plant species. Plants of the Bromeliad family, which includes pineapples, start to flower if treated with IAA. Some other plants typically produce flowers that can develop as either solely male or solely female flowers depending on various environmental factors: In several such species IAA stimulates femaleness.

Gibberellins (GAs).

Produced in young leaves, developing seeds, and probably in root tips, the biologically active GAs, such as GA1 and GA3, move in shoots without polarity and at a slower rate than IAA down the stems where they cause elongation. In roots they show root-tip toward root-base polar movementthe opposite of IAA. Their effect on stem elongation is particularly striking in some plants that require exposure to long days in order to flower. In such plants the stem elongation that precedes flowering is caused by either long days or active GAs and is so fast that it is called bolting. A similar association of light effects and active GAs is found in seeds that normally require light or cold treatment to germinate. GAs can substitute for these environmental treatments. In cereal seeds, GA, produced by the embryos, moves into the parts of the seeds containing starch and other storage products. There the GA triggers the production of various specific enzymes such as alpha-amylase, which breaks down starch into smaller compounds usable by the growing embryos. In the flowers that can develop as either male or female, active GAs cause maleness (the opposite effect to that of auxin). Not surprisingly, in view of the relatively large amounts of GAs in seeds, spraying GAs on such seedless grape varieties as Thompson produces bigger and more elongated grapes on the vines.

Cytokinins.

Produced in roots and seeds, the cytokinins' often-reported presence in leaves apparently results from accumulation of cytokinins produced by roots and moved to the shoot through the xylem cells. Research using pieces of plant tissue growing in test tubes revealed that adding cytokinins increased cell divisions and subsequently the number of shoot buds that regenerated, while increasing the amount of added IAA increased the number of roots formed. The test-tube cultures could be pushed toward bud or root formation by changing the ratio of cytokinin to IAA. The growth of already-formed lateral buds on stems could be stimulated in some plants by treating the lateral buds directly with cytokinins. With IAA from the apex of the main shoot inhibiting outgrowth of the lateral buds and with cytokinins stimulating their outgrowth, the effects of the two hormones on lateral buds suggests a balancing effect like that seen in root/shoot regeneration in the tissue cultures. Treatment with cytokinins retards the senescence of leaves, and naturally occurring leaf senescence is accompanied by a decrease in native cytokinins. When the movement of cytokinins such as zeatin through excised petioles was tested in the same sort of experiment that showed IAA moving with polarity at 5 to 10 millimeters per hour, cytokinins showed the slower rate of movement and the lack of polarity characteristic of GAs. However, through root sections, zeatin movement was nonpolar , unlike the movement of GAs.

Abscisic Acid.

Abscisic acid is found in leaves, roots, fruits, and seeds. In leaves that are not wilting, the hormone is mostly in the chloroplasts . When wilting starts the abscisic acid is released for movement to the guard cells of the stomates. Abscisic acid moves without polarity through stem sections and at the slower rate typical of GAs and cytokinins.

As its name implies, abscisic acid stimulates leaf or fruit abscission in many species, as evidenced by faster abscission from treating with the hormone and by increases in the amount of native abscisic acid in cotton fruits just prior to their natural abscission. Abscisic acid's most investigated effect, however, is its protection of plants from too much water loss (wilting) by closing the stomates in leaves when wilting starts. The onset of wilting is accompanied by fast increases in the abscisic acid levels in the leaves and subsequent closure of the stomates. Spraying the leaves with abscisic acid causes stomate closure even if the leaves are not wilting. In seeds, abscisic acid prevents premature germination of the seed.

Ethylene Gas.

Ethylene gas is produced by many parts of plants when they are stressed. Also, normally ripening fruits are often rich producers of ethylene. Among ethylene's many effects are speeding the ripening of fruits and the senescence and abscission of leaves and flower parts; indeed, it is used commercially to coordinate ripening of crops to make harvesting more efficient. Ethylene gas releases seeds from dormancy. If given as a pretreatment, it inhibits the polar movement of auxin in stems of land plants (but, surprisingly, increases auxin movement in some plants that normally grow in fresh water). Ethylene moves readily through and out of the plant. The stimulation of flowering in pineapple and other bromeliads by spraying with IAA, mentioned earlier, is due to ethylene produced by the doses of auxin applied. Despite its frequent production by plants, ethylene is apparently not essential for plant development. Mutations or chemicals that block ethylene production do not prevent normal development.

Interactions of Hormones

In addition to the many effects on development of individual plant growth hormones, a sizeable number of effects of one hormone on another have been found. For example, IAA alone can restore the full number of normal tracheary cells in the xylem, but to restore the full number of sieve-tube cells in the phloem zeatin is needed in addition to IAA. Similarly, to restore the full number of fibers in the phloem, GA must be added along with IAA.

Hormones affect each other's movement, too. Mentioned above was the decrease in IAA movement from pretreatment with ethylene. Similarly, abscisic acid decreases the basipetal polar movement of IAA in stems and petioles. Therefore, in view of IAA's role as the primary inhibitor of abscission in plants, the abscisic acid-induced decrease in IAA movement down the leaf stalk toward the abscision zone probably explains at least part of abscisic acid's role as an accelerator of abscission. In other cases, increases in IAA basipetal movement have resulted from GA or cytokinin treatment. The nonpolar movement typical of cytokinins was changed to polar movement when IAA was added, too.

see also Differentiation and Development; Embryogenesis; Genetic Mechanisms and Development; Germination and Growth; Hormonal Growth and Development; Photoperiodism; Seedless Vascular Plants; Senescence; Tropisms.

William P. Jacobs

Bibliography

Abeles, Frederick B., Page W. Morgan, and Mikal E. Saltveit, Jr. Ethylene in Plant Biology, 2nd ed. San Diego, CA: Academic Press, 1992.

Addicott, Fredrick T., ed. Abscisic Acid. New York: Praeger Publishers, 1983.

Davies, Peter J., ed. Plant Hormones: Physiology, Biochemistry, and Molecular Biology. Boston: Kluwer Academic Pulishers, 1995.

Jacobs, William P. Plant Hormones and Plant Development. Cambridge, England: Cambridge University Press, 1979.

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hormone

hormone
1. A substance that is manufactured and secreted in very small quantities into the bloodstream by an endocrine gland or a specialized nerve cell (see neurohormone) and regulates the growth or functioning of a specific tissue or organ in a distant part of the body. For example, the hormone insulin controls the rate and manner in which glucose is used by the body. Other hormones include the sex hormones, corticosteroids, adrenaline, thyroxine, and growth hormone.

2. A plant growth substance.

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hormone

hor·mone / ˈhôrˌmōn/ • n. Physiol. a regulatory substance produced in an organism and transported in tissue fluids such as blood or sap to stimulate specific cells or tissues into action. ∎  a synthetic substance with a similar effect. ∎  (hormones) a person's sex hormones as held to influence behavior or mood. DERIVATIVES: hor·mo·nal / hôrˈmōnl/ adj.

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hormone

hormone (hor-mohn) n. a substance that is produced by an endocrine gland in one part of the body, passes into the bloodstream, and is carried to other (distant) organs or tissues, where it acts to modify their structure or function. Examples of hormones are corticosteroids, adrenaline, growth hormone, androgens, oestrogens, thyroid hormone, and insulin.

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hormone

hormone A regulatory substance, active at low concentrations, that is produced in specialized cells but that exerts its effect either on distant cells or on all cells in the organism to which it is conveyed via tissue fluids.

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hormone

hormone A regulatory substance, active at low concentrations, that is produced in specialized cells but that exerts its effect either on distant cells or on all cells in the organism to which it is conveyed via tissue fluids.

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hormones

hormones Compounds produced in the body in endocrine glands, and released into the bloodstream, where they act as chemical messengers to affect other tissues and organs.

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hormone

hormonealone, atone, Beaune, bemoan, blown, bone, Capone, clone, Cohn, Cologne, condone, cone, co-own, crone, drone, enthrone, flown, foreknown, foreshown, groan, grown, half-tone, home-grown, hone, Joan, known, leone, loan, lone, moan, Mon, mown, ochone, outflown, outgrown, own, phone, pone, prone, Rhône, roan, rone, sewn, shown, Simone, Sloane, Soane, sone, sown, stone, strown, throne, thrown, tone, trombone, Tyrone, unbeknown, undersown, zone •Dione • backbone • hambone •breastbone • aitchbone •tail bone, whalebone •cheekbone • shin bone • hip bone •wishbone • splint bone • herringbone •thigh bone • jawbone • marrowbone •knuckle bone • collarbone •methadone • headphone • cellphone •heckelphone • payphone • Freefone •radio-telephone, telephone •videophone • francophone •megaphone • speakerphone •allophone • Anglophone • xylophone •gramophone • homophone •vibraphone • microphone •saxophone • answerphone •dictaphone •sarrusophone, sousaphone •silicone • pine cone • snow cone •flyblown • cyclone • violone •hormone • pheromone • Oenone •chaperone • progesterone •testosterone

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Hormones

Hormones

Mechanisms of action

Major hormones

The hypothalamus

The pituitary gland

The thyroid gland

The parathyroid glands

The adrenal glands

The pancreas

The female reproductive organs

The male reproductive organs

Resources

Hormones are biochemical messengers that regulate physiological events in living organisms. More than 100 hormones have been identified in humans. Hormones are secreted by endocrine (ductless) glands such as the hypothalamus, the pituitary gland, the pineal gland, the thyroid, the parathyroid, the thymus, the adrenals, the pancreas, the ovaries, and the testes. Hormones are secreted directly into the blood stream from where they travel to target tissues and modulate digestion, growth, maturation, reproduction, and homeostasis.

The word hormone comes from the Greek word hormon, which means to stir up. Indeed, excitation is characteristic of the adrenaline and the sex hormones. Most hormones produce an affect on specific target tissues that are sited at some distance from the gland secreting the hormone. Although small plasma concentrations of most hormones are always present, surges in secretion trigger specific responses at one or more targets. Hormones do not fall into any one chemical category, but most are either protein molecules or steroid molecules. These biological managers keep the body systems functioning over the long term and help maintain health. The study of hormones is called endocrinology.

Mechanisms of action

Hormones elicit a response at their target tissue, target organ, or target cell type through receptors. Receptors are molecular complexes that specifically recognize another moleculein this case, a particular hormone. When the hormone is bound by its receptor, the receptor is usually altered in some way that it sends a secondary message through the cell to do something in response. Hormones that are proteins, or peptides (smaller strings of amino acids), usually bind to a receptor in the cells outer surface and use a second messenger to relay their action. Steroid hormones such as cortisol, testosterone, and estrogen bind to receptors inside cells. Steroids are small enough to and chemically capable of passing through the cells outer membrane. Inside the cell, these hormones bind their receptors and often enter the nucleus to elicit a response. These receptors bind DNA (deoxyribonucleic acid) to regulate cellular events by controlling gene activity.

Most hormones are released into the bloodstream by a single gland. Testosterone is an exception, because it is secreted by both the adrenal glands and by the testes. Plasma concentrations of all hormones are assessed at some site that has receptors binding that hormone. The site keeps track of when the hormone level is low or high. The major area that records this information is the hypothalamus. A number of hormones are secreted by the hypothalamus that stimulate or inhibit additional secretion of other hormones at other sites. The hormones are part of a negative or positive feedback loop.

Most hormones work through a negative feedback loop. As an example, when the hypothalamus detects high levels of a hormone, it reacts to inhibit further production. And when low levels of a hormone are detected, the hypothalamus reacts to stimulate hormone production or secretion. Estrogen, however, is part of a positive feedback loop. Each month, the Graafian follicle in the ovary releases estrogen into the bloodstream as the egg develops in ever increasing amounts. When estrogen levels rise to a certain point, the pituitary secretes luteinizing hormone (LH), which triggers the eggs release of the egg into the oviduct.

Not all hormones are readily soluble in blood (their main transport medium) and require a transport molecule that will increase their solubility and shuttle them around until they get to their destination. Steroid hormones, in particular, tend to be less soluble. In addition, some very small peptides require a carrier protein to deliver them safely to their destination, because these small peptides could be swept into the wrong location where they would not elicit the desired response. Carrier proteins in the blood include albumin and prealbumin. There are also specific carrier proteins for cortisol, thyroxin, and the steroid sex hormones.

Major hormones

The concentrations of several important biological building blocks such as amino acids are regulated by more than one hormones. For example, both calcitonin and parathyroid hormone (PTH) influence blood calcium levels directly, and other hormones affect calcium levels indirectly via other pathways.

The hypothalamus

Hormones secreted by the hypothalamus modulate other hormones. The major hormones secreted by the hypothalamus are corticotrophin releasing hormone (CRH), thyroid stimulating hormone releasing hormone (TRH), follicle stimulating hormone releasing hormone (FSHRH), luteinizing hormone releasing hormone (LRH), and growth hormone releasing hormone (GHRH). CRH targets the adrenal glands. It triggers the adrenals to release adrenocorticotropic hormone (ACTH). ACTH functions to synthesize and release corticosteroids. TRH targets the thyroid where it functions to synthesize and release the thyroid hormones T3 and T4. FSH targets the ovaries and the testes where it enables the maturation of the ovum and of spermatozoa. LRH also targets the ovaries and the testes, and its receptors are in cells that promote ovulation and increase progesterone synthesis and release. GHRH targets the anterior pituitary to release growth hormones to most body tissues, increase protein synthesis, and increase blood glucose. Hence, the hypothalamus plays a first domino role in these cascades of events.

The hypothalamus also secretes some other important hormones such as prolactin inhibiting hormone (PIH), prolactin releasing hormone (PRH), and melanocyte inhibiting hormone (MIH). PIH targets the anterior pituitary to inhibit milk production at the mammary gland, and PRH has the opposite effect. MIH targets skin pigment cells (melanocytes) to regulate pigmentation.

The pituitary gland

The pituitary has long been called the master gland because of the vast extent of its activity. It lies deep in the brain just behind the nose. The pituitary is divided into anterior and posterior regions with the anterior portion comprising about 75% of the total gland. The posterior region secretes the peptide hormones vasopressin, also called anti-diuretic hormone (ADH), and oxytocin. Both are synthesized in the hypothalamus and moved to the posterior pituitary prior to secretion. ADH targets the collecting tubules of the kidneys, increasing their permeability to water. ADH causes the kidneys to retain water. Lack of ADH leads to a condition called diabetes insipidus characterized by excessive urination. Oxytocin targets the uterus and the mammary glands in the breasts. Oxytocin begins labor prior to birth and also functions in the ejection of milk. The drug, pitocin, is a synthetic form of oxytocin and is used medically to induce labor.

The anterior pituitary (AP) secretes a number of hormones. The cells of the AP are classified into five types based on what they secrete. These cells are somatotrophs, corticotrophins, thyrotrophs, lactotrophs, and gonadotrophs. Respectively, they secrete growth hormone (GH), ACTH, TSH, prolactin, and LH and FSH. Each of these hormones is either a polypeptide or a glycoprotein. GH controls cellular growth, protein synthesis, and elevation of blood glucose concentration. ACTH controls secretion of some hormones by the adrenal cortex (mainly cortisol). TSH controls thyroid hormone secretion in the thyroid. In males, prolactin enhances testosterone production; in females, it initiates and maintains LH to promote milk secretion from the mammary glands. In females, FSH initiates ova development and induces ovarian estrogen secretion. In males, FSH stimulates sperm production in the testes. LH stimulates ovulation and formation of the corpus luteum which produces progesterone. In males, LH stimulates interstitial cells to produce testosterone. Each AP hormone is secreted in response to a hypothalamic releasing hormone.

The thyroid gland

The thyroid lies under the larynx and synthesizes two hormones, thyroxine and tri-iodothyronine. This gland takes up iodine from the blood and has the highest iodine level in the body. The iodine is incorporated into the thyroid hormones. Thyroxine has four iodine atoms and is called T4. Tri-iodothyronine has three iodine atoms and is called T3. Both T3 and T4 function to increase the metabolic rate of several cells and tissues. The brain, testes, lungs, and spleen are not affected by thyroid hormones, however. T3 and T4 indirectly increase blood glucose levels as well as the insulin-promoted uptake of glucose by fat cells. Their release is modulated by TSH-RH from the hypothalamus. TSH secretion increases in cold infants. When temperature drops, a metabolic increase is triggered by TSH. Chronic stress seems to reduce TSH secretion that, in turn, decreases T3 and T4 output.

Depressed T3 and T4 production is the trademark of hypothyroidism. If it occurs in young children, then this decreased activity can cause physical and mental retardation. In adults, it creates sluggishnessmentally and physicallyand is characterized further by weight gain, poor hair growth, and a swollen neck. Excessive T3 and T4 cause sweating, nervousness, weight loss, and fatigue. The thyroid also secretes calcitonin that serves to reduce blood calcium levels. Calcitonins role is particularly significant in children whose bones are still forming.

The parathyroid glands

The parathyroid glands are attached to the bottom of the thyroid gland. They secrete the polypeptide parathyroid hormone (PTH) which plays a crucial role in monitoring blood calcium and phosphate levels. About 99% of the bodys calcium is in the bones, and 85% of the magnesium is also found in bone. Low blood levels of calcium stimulate PTH release into the bloodstream in two steps. Initially, calcium is released from the fluid around bone cells. And later, calcium can be drawn from bone itself. However, only about 1% of bone calcium is readily exchangeable. PTH can also increase the absorption of calcium in the intestines by stimulating the kidneys to produce a vitamin D-like substance which facilitates this action. High blood calcium levels will inhibit PTH action, and magnesium (which is chemically similar to calcium) shows a similar effect.

Calcium is a critical element for the human body. Even though the majority of calcium is in bone, it is also used by muscles, including cardiac muscle for contractions, and by nerves in the release of neurotransmitters. Calcium is a powerful messenger in the immune response of inflammation and blood clotting. Both PTH and calcitonin regulate calcium levels in the kidneys, the gut, bone, and blood. Whereas calcitonin is released in conditions of high blood calcium levels, PTH is released when calcium levels fall in the blood. Comparing the two, PTH causes an increase in calcium absorption in the kidneys, absorption in the intestine, release from bone, and levels in the blood. In addition, PTH decreases kidney phosphate absorption. Calcitonin has the opposite effect on each of these variables. PTH is thought to be the major calcium modulator in adults.

PTH deficiency can be due to autoimmune diseases or to inherited parathyroid gland problems. Low PTH capabilities cause depressed blood calcium levels and neuromuscular problems. Very low PTH can lead to tetany or muscle spasms. Excess PTH can lead to weakened bones because it causes too much calcium to be drawn from the bones and to be excreted in the urine. Abnormalities of bone mineral deposits can lead to a number of conditions including osteoporosis and rickets. Osteoporosis can be due to dietary insufficiencies of calcium, phosphate, or vitamin C (which has an important role in formation of the bone matrix). The end result is a loss of bone mass. Rickets is usually caused by a vitamin D deficiency and results in lower rates of bone matrix formation in children. These examples show how important a balanced nutritious diet is for healthy development.

The adrenal glands

The two adrenal glands, one on top of each kidney, each have two distinct regions. The outer region (the medulla) produces adrenaline and noradrenaline and is under the control of the sympathetic nervous system. The inner region (the cortex) produces a number of steroid hormones. The cortical steroid hormones include mineralocorticoids (mainly aldosterone), glucocorticoids (mainly cortisol), and gonadocorticoids. These steroids are derived from cholesterol. Although cholesterol receives plenty of bad publicity, some amount of cholesterol is necessary. Steroid hormones act by regulating gene expression, hence, their presence controls the production of numerous factors with multiple roles. Aldosterone and cortisol are the major human steroids in the cortex. However, testosterone and estrogen are secreted by adults (both male and female) at very low levels.

Aldosterone plays an important role in regulating body fluids. It increases blood levels of sodium and water and lowers blood potassium levels. Low blood sodium levels trigger aldosterone secretion via the renin-angiotensin pathway. Renin is produced by the kidney, and angiotensin originates in the liver. High blood potassium levels also trigger aldosterone release. ACTH has a minor promoting effect on aldosterone. Aldosterone targets the kidney where it promotes sodium uptake and potassium excretion. Since sodium ions influence water retention, the result is a net increase in body fluid volume.

Blood cortisol levels fluctuate dramatically throughout the day and peak around 8 a.m. Presumably, this early peak enables humans to face the varied daily stressors they encounter. Cortisol secretion is stimulated by physical trauma, cold, burns, heavy exercise, and anxiety. Cortisol targets the liver, skeletal muscle, and adipose tissue. Its overall effect is to provide amino acids and glucose to meet synthesis and energy requirements for normal metabolism and during periods of stress. Because of its anti-inflammatory action, it is used clinically to reduce swelling. Excessive cortisol secretion leads to Cushing syndrome that is characterized by weak bones, obesity, and a tendency to bruise. Cortisol deficiency can lead to Addison disease, which has the symptoms of fatigue, low blood sodium levels, low blood pressure, and excess skin pigmentation.

The adrenal medullary hormones are epinephrine (adrenaline) and nor-epinephrine (nor-adrenaline). Both of these hormones serve to supplement and prolong the fight or flight response initiated in the nervous system. This response includes the neural effects of increased heart rate, peripheral blood vessel constriction, sweating, spleen contraction, glycogen conversion to glucose, dilation of bronchial tubes, decreased digestive activity, and lowered urine output.

The condition of stress presents a model for reviewing one way that multiple systems and hormones interact. During stress, the nervous, endocrine, digestive, urinary, respiratory, circulatory, and immune response are all tied together. For example, the hypothalamus sends nervous impulses to the spinal cord to stimulate the fight or flight response and releases CRH that promotes ACTH secretion by the pituitary. ACTH, in turn, triggers interleukins to respond, which promote immune cell functions. ACTH also stimulates cortisol release at the adrenal cortex, which helps buffer the person against stress. As part of a negative feedback loop, ACTH and cortisol receptors on the hypothalamus assess when sufficient levels of these hormones are present and then inhibit their further release. De-stressing occurs over a period of time after the stressor is gone. The systems eventually return to normal.

The pancreas

The pancreas folds under the stomach, secretes the hormones insulin, glucagon, and somatostatin. About 70% of the pancreatic hormone-secreting cells are called beta cells and secrete insulin; another 22%, or so, are called alpha cells and secrete glucagon. The remaining gamma cells secrete somatostatin, also known as growth hormone inhibiting hormone (GHIH). The alpha, beta, and gamma cells comprise the islets of Langerhans that are scattered throughout the pancreas.

Insulin and glucagon have reciprocal roles. Insulin promotes the storage of glucose, fatty acids, and amino acids, whereas, glucagon stimulates mobilization of these constituents from storage into the blood. Both are relatively short polypeptides. Insulin release is triggered by high blood glucose levels. It lowers blood sugar levels by binding a cell surface receptor and accelerating glucose transport into the cell where glucose is converted into glycogen. Insulin also inhibits the release of glucose by the liver in order to keep blood levels down. Increased blood levels of GH and ACTH also stimulate insulin secretion. Not all cells require insulin to store glucose, however. Brain, liver, kidney, intestinal, epithelium, and the pancreatic islets can take up glucose independently of insulin. Insulin excess can cause hypoglycemia leading to convulsions or coma, and insufficient levels of insulin can cause diabetes mellitus that can be fatal if left untreated. Diabetes mellitus is the most common endocrine disorder.

Glucagon secretion is stimulated by decreased blood glucose levels, infection, cortisol, exercise, and large protein meals. GHIH, glucose, and insulin inhibit its secretion. Protein taken in through the digestive tract has more of a stimulatory effect on glucagon than does injected protein. Glucagon stimulates glycogen breakdown in the liver, inhibits glycogen synthesis, and facilitates glucose release into the blood. Excess glucagon can result from tumors of the pancreatic alpha cells; and a mild diabetes seems to result. Some cases of uncontrolled diabetes are also characterized by high glucagon levels suggesting that low blood insulin levels are not always the only cause in some diabetes cases.

It was the study of glucagon and its action by Sutherland in 1961 that led to the concept of the second messenger system. Glucagon activates the intracellular molecule cyclic AMP, cAMP. Since this discovery, a number of other molecules have been found that modulate cellular activity via this second messenger.

The female reproductive organs

The female reproductive hormones arise from the hypothalamus, the anterior pituitary, and the ovaries. Although detectable amounts of the steroid hormone estrogen are present during fetal development, at puberty estrogen levels rise to initiate secondary sexual characteristics. Gonadotropin releasing hormone (GRH) is released by the hypothalamus to stimulate pituitary release of LH and FSH. LH and FSH propagate egg development in the ovaries. Eggs (ova) exist at various stages of development, and the maturation of one ovum takes about 28 days and is called the ovarian or menstrual cycle. The ova are contained within follicles, which are support organs for ova maturation. About 450 of a females 150,000 germ cells mature to leave the ovary. The hormones secreted by the ovary include estrogen, progesterone, and small amounts of testosterone.

As an ovum matures, rising estrogen levels stimulate additional LH and FSH release from the pituitary. Prior to ovulation, estrogen levels drop, and LH and FSH surge to cause the ovum to be released into the fallopian tube. The cells of the burst follicle begin to secrete progesterone and some estrogen. These hormones trigger thickening of the uterine lining, the endometrium, to prepare it for implantation should fertilization occur. The high progesterone and estrogen levels prevent LH and FSH from further secretionthus hindering another ovum from developing. If fertilization does not occur, eight days after ovulation the endometrium deteriorates resulting in menstruation. The falling estrogen and progesterone levels, which follow trigger LH and FSH, start the cycle all over again.

Although estrogen and progesterone have major roles in the menstrual cycle, these hormones have receptors on a number of other body tissues. Estrogen has a protective effect on bone loss that can lead to osteoporosis. And progesterone, which is a competitor for androgen sites, blocks actions that would result from testosterone activation. Estrogen receptors have even been found in the forebrain indicating a role in female neuronal function or development.

Hormones related to pregnancy include human chorionic gonadotrophin (HCG), estrogen, human chorionic somatomammotrophin (HCS), and relaxin. HCG is released by the early embryo to signal implantation. Estrogen and HCS are secreted by the placenta. And relaxin is secreted by the ovaries as birth nears to relax the pelvic area in preparation for labor.

The male reproductive organs

Male reproductive hormones come from the hypothalamus, the anterior pituitary, and the testes. As in females, GRH is released from the hypothalamus that stimulates LH and FSH release from the pituitary. In males, LH and FSH facilitate spermatogenesis. The steroid hormone testosterone is secreted from the testes and can be detected in early embryonic development up until shortly after birth. Testosterone

KEY TERMS

Amino acid An organic compound whose molecules contain both an amino group (-NH2) and a carboxyl group (-COOH). One of the building blocks of a protein.

Androgen Any hormone with testosterone-like activity (i.e. it increases male characteristics).

Homeostasis A condition of chemical and physical equilibrium in the human body.

Plasma The non-cellular, fluid portion of blood in which the concentration of most molecules is measured.

levels are quite low until puberty. At puberty, rising levels of testosterone stimulate male reproductive development including secondary characteristics.

LH stimulates testosterone release from the testes. FSH promotes early spermatogenesis, whereas testosterone is required to complete spermatogenic maturation to facilitate fertilization. In addition to testosterone, LH, and FSH, the male also secretes prostaglandins. These substances promote uterine contractions that help propel sperm towards an egg in the fallopian tubes during sexual intercourse. Prostaglandins are produced in the seminal vesicles, and are not classified as hormones by all authorities.

See also Biological rhythms; Cell; Endocrine system; Exocrine glands; Glands; Growth hormones; Reproductive system.

Resources

BOOKS

Burnstein, K. L. Steroid Hormones and Cell Cycle Regulation. Boston, MA: Kluwer Academic Publishers, 2002.

Germann, William J. Principles of Human Physiology. San Francisco, CA: Pearson Benjamin Cummings, 2005.

Goffin, V., Kelly, P.A. Hormone Signaling Boston, MA: Kluwer Academic Publishers, 2002.

Greenstein, Ben. The Endocrine System at a Glance. Malden, MA: Blackwell Publishing, 2006.

Rushton, Lynette. The Endocrine System. Philadelphia, PA: Chelsea House Publishers, 2004.

Van De Graaff, Kent M., and R. Ward Rhees, eds. Human Anatomy and Physiology: Based on Schaums Outline of Theory and Problems of Human Anatomy and Physiology. New York: McGraw-Hill, 2001.

Louise Dickerson

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Hormones

Hormones

This entry contains the following:

I. OVERVIEW
Diane Sue Saylor

II. SEX HORMONES
Diane Sue Saylor

III. HORMONAL DISEASES
Diane Sue Saylor

I. OVERVIEW

A hormone (from the Greek hormaein, "to excite") is a chemical substance that is produced by a single cell or a group of cells and is secreted directly into the bloodstream where it may circulate through the body or act locally. Hormones regulate or control other cells or physiological systems in the body, including organs and biological systems that deal with sexual behavior and reproduction. Hormones act as messengers to initiate chemical reactions that may have either a specific local effect on cells in the general vicinity of the tissue that secretes them (such as acetylcholine, which stimulates the muscle immediately next to the nerve fibers that release it) or a more diffuse effect, as with general hormones that are carried through the bloodstream and exert their influence on remote areas of the body. The second category of these compounds may affect all or nearly all the cells and organs systems of the body or may affect only specific cells or organs.

THE FORMATION AND RELEASE OF HORMONES

General hormones usually are secreted by a specific endocrine gland: a collection of specialized tissue that produces and releases hormones into the bloodstream without the use of ducts or tubes, in contrast to exocrine glands which use a duct system to distribute their secretions. From the bloodstream the chemical messengers attach to specific target receptors. Depending on where those receptors are in the body, the target sites determine the area of influence of a hormone. For example, growth hormone, which comes from the anterior pituitary gland, reacts with most of the cells in the body. In contrast, many general hormones work only on target tissues that have specific receptors to bind to a particular hormone and initiate its action. Ovarian hormones are an example of this type in that they are manufactured in the ovaries and released into the circulatory system; from there they attach themselves to cells and organs that control the primary and secondary sexual characteristics of females, including the reproductive system and breast development. Most general hormones are secreted by endocrine glands.

Examples of endocrine tissue that produces hormones include the pituitary gland (which makes growth hormone, thyroid-stimulating hormone, follicle-stimulating hormone, luteinizing hormone, prolactin, and oxytocin), the thyroid gland (thyroxin and calcitonin), the adrenal glands (cortisol and aldosterone), the pancreas (insulin), the ovaries (estrogens and progestins), the testes (testosterone), and the placenta (human chorionic gonadotro-pin, estrogens, and progesterone).

Endocrine glands make hormones by the process of anabolism: the formation of complex chemical compounds from smaller particles. Hormones fall into one of three chemical categories: steroids, derivatives of tyrosine (an amino acid), and proteins or peptides. Steroids are of particular importance in human sexuality and reproduction and include the hormones produced by the ovaries, the testes, the adrenal glands, and the placenta. Tyrosine derivatives include the major thyroid hormones, epinephr-ine, and norepinephrine. Protein or peptide hormones include oxytocin as well as hormones that deal with kidney function (vasopressin) and blood sugar levels (insulin).

Protein hormones generally are formed at the subcellular level, in the endoplasmic reticulum. The chemical created (the preprohormone) is usually much larger than the final hormone and must undergo a process of cleaving to make a smaller protein called a prohormone. That compound undergoes one more cleaving to make the final hormone. Those hormones often are encapsulated in small vesicles called secretory vesicles or granules and are stored in the cytoplasm of the endocrine cell until there is a signal for their release into the bloodstream. The amount of steroid hormones stored in the cytoplasm of ovarian or testicular cells is usually very small, but the precursor building blocks (especially cholesterol) are present in great numbers. Within minutes of the specific stimulation, enzymes in those cells assemble the precursors into the final hormones and release them into the blood.

THE ACTIONS OF HORMONES

Hormones work principally by regulating body metabolic functions through chemical reactions at the cellular level. Those reactions and their effects may occur within seconds of the release of a hormone or may take up to several weeks (as with thyroid hormones, which may take weeks to affect metabolism) or years to have an effect. These compounds occur in minute quantities in the bloodstream are released in an as-needed manner that is apparently adequate to regulate and control most metabolic functions. They generally are released into the body in a rate determined by negative feedback; that is, the endocrine gland almost always naturally oversecretes its hormone, which in turn exerts control over the target organ, which then performs its function. Some mechanism informs the endocrine gland that no more hormone is needed, and that negative feedback causes the gland to decrease its rate of secretion. When the target organ does not function, it does not send a signal to limit production, and so the endocrine gland steps up production and secretion of the hormone even if that is harmful to the organ.

Hormones almost never react directly with the target cell machinery but combine with hormone receptors that trigger a change in the target cell, for example, by changing the permeability of the target cell membrane (as with epinephrine) and thus exciting it to action or inhibiting its function. Often stimulation of the receptor cell activates an intracellular reaction that results in the production of a second messenger that then institutes cellular changes (as with insulin). Other hormonal targets activate protein receptors inside the cell and thus form a hormone-receptor complex that initiates the activation of specific genes to form new proteins in the cell (and thus change the cell's metabolism). That mechanism characterizes the manner in which thyroid hormones and steroids (including the sex hormones) work. Hormones act by initiating a cascade of reactions in the cell. This is one reason so little hormonal stimulation is necessary to yield an effect. Hormones are necessary elements for human growth, metabolism, and sexuality, including both primary and secondary sexual characteristics and reproduction.

BIBLIOGRAPHY

Berne, Robert M., et al., eds. 1998. Physiology. 4th edition. St. Louis, MO: Mosby.

Guyton, Arthur C. 1991. Textbook of Medical Physiology. 8th edition. Philadelphia: Saunders.

                                           Diane Sue Saylor

II. SEX HORMONES

Hormones play an integral role in human sexuality. They help in the formation of the primary sexual characteristics (including sex organs such as the uterus and the penis) and the secondary sexual characteristics (such as the growth of breasts and pubic hair) and influence sexual behavior. Along with genetic material, neurological stimuli, and social influences, hormones are responsible for determining the gender of males and females.

HORMONES IN REPRODUCTION IN MALES

Collectively, the hormones secreted by the testes are called androgens (steroid hormones that have masculinizing effects). Other areas of the body (such as the adrenal glands) produce androgens but in quantities so minute that they have little masculinizing effect on men or women except to cause pubic and axillary hair growth. Testosterone, the most important male hormone, is responsible for causing the body to develop male sexual characteristics.

After the onset of puberty, testosterone causes the penis, scrotum, and testes to increase in size. That hormone also stimulates the secondary sexual characteristics, which include body hair distribution (including pubic, facial, chest, and other body hair), baldness (testosterone and the genetic tendency to become bald work together to cause male pattern hair loss at the temples and the crown of the head), deepening of the voice (usually starting during puberty with a "cracking" voice that swings quickly from low to high pitch), thickening of the skin, development of acne (though this tends to diminish after a few years), increased muscular development (averaging around 50 percent more than that of females), and an increase in the calcium density of bones. In addition, testosterone causes the male pelvis to develop in a uniquely male pattern: longer and narrower and able to bear more weight than the female pelvis. Less obvious characteristics include an increase in the metabolic rate (the amount of energy produced and used by the body over time), an increase in the number of red blood cells, and an increase in the ability of the kidneys to reabsorb sodium, which allows for larger fluid and blood volumes in relation to weight.

Hormones also work with the central nervous system and to regulate male sexual behavior. Although the mechanism of this process is not completely understood, male sex drive (libido) and male behavior are heavily dependent on testosterone and its related compounds, though the effect of androgens on erections in men is context-sensitive; that is, it requires both physical and psychological stimulation to elicit a penile response. The presence of testosterone can increase aggression, though social variables play a large role in mitigating agonistic behavior. The effect of testosterone on sex drive and aggressive behavior is controversial, with some researchers suggesting that the sex hormones play a lesser role than do social influences.

HORMONES IN FETAL AND ADOLESCENT DEVELOPMENT

By the seventh week of fetal development (gestation) hormones from the placenta (especially human chorionic gonadotropin [HCG]) stimulate the fetal testes to produce moderate amounts of testosterone that remain throughout intrauterine development and last until about ten weeks after birth. This early testosterone production is responsible for the development of the male body characteristics, including the formation of the external male genitalia (penis and scrotum) and internal reproductive organs (such as the prostate gland, the seminal vesicles, and the male genital ducts). In addition, the increased quantity of androgens suppresses the formation of female genitalia and reproductive organs, which all fetuses will develop in the absence of testosterone. Sufficient levels of testosterone are necessary to stimulate the testes to descend into the scrotal sac during the last couple of months of pregnancy, although when this does not happen, the therapeutic administration of testosterone or other gonadotropic hormones is sometimes effective in getting the testes to descend.

After birth the testes essentially do not produce any testosterone until the onset of puberty (about age ten to thirteen). At that point the anterior pituitary begins to secrete hormones that cause an increase in the production of testosterone in the testes. Testosterone production continues throughout a male's life but begins to diminish after middle age.

MALE HORMONE PRODUCTION

Although most of the masculinizing hormones are produced locally in the testes, they require other hormones that are formed in other parts of the body to stimulate their secretion into the body. Luteinizing hormone (LH) is formed in the anterior pituitary gland near the brain and is secreted into bloodstream, where it activates receptors in the Leydig cells in the testes. This stimulates the cells to produce the hormone testosterone, a necessary component in spermatogenesis (the production of sperm, the male reproductive cell). Follicle-stimulating hormone (FSH) also is secreted by the anterior pituitary gland and stimulates the Sertoli cells in the seminiferous tubules (tubules within the lobules of the testes) to convert spermatids, a precursor form of the male sex reproductive cell, into mature sperm. Small amounts of estrogens (female hormones) also are formed during FSH stimulation of the Sertoli cells and probably are necessary for the formation of sperm. The Sertoli cells produce an androgen-binding protein that attaches to both estrogen and testosterone allowing them to pass into the canal of the seminiferous tubule and bringing the hormones into contact with the developing sperm. Other, more generalized hormones, including growth hormone, are necessary not only for promoting the development of the early stages of sperm production but also for the general metabolic functions of the testes.

ROLE OF HORMONES IN REPRODUCTION IN FEMALES

Hormones help the female reproductive system in its two major phases: preparing the body for its role in conception and gestation (the period of development of the fertilized egg until birth). Hormones that control and regulate the female reproductive system include gonadotropin-releasing hormone (GnRH), also called luteinizing hormone-releasing hormone (LHRH), which is released by the hypothalamus in the brain. Its circulation into the bloodstream in turn causes the anterior pituitary to release follicle-stimulating hormone and luteinizing hormone, which in turn initiates the secretion of the two main types of ovarian hormones: estrogens (the most important of which is estradiol) and progestins (the most important of which is progesterone).

The main function of estrogen is to promote the growth of the cells and tissues responsible for the secondary sexual characteristics of the female. Progestins function mainly in preparing the uterus for pregnancy and the breasts for lactation (milk production). Estrogen also causes an increase in the growth of bones, causing females to enter into a growth spurt at puberty. The same hormone also causes the long bones in women to fuse and stop growing several years earlier than is the case for their male counterparts. Estrogens increase the metabolic rate of the body (though not as much as testosterone does) and cause deposition of fat in the breasts, buttocks, and thighs. Increased concentrations of estrogen, as in pregnancy, can cause water retention. Estrogens make the skin soft and smooth and increase its vascularity. All these feminizing hormones are secreted at differing rates, depending on what phase the woman is in her monthly menstrual cycle (also called the female sexual cycle).

Females also produce androgens (masculinizing hormones). Testosterone-like hormones secreted by the adrenal gland along with estrogen from the ovaries affect target cells in the brain to influence sexual behavior and libido. These hormones work in conjunction with neural and psychological factors to affect female sexuality and behavior (as they also do in males) in a manner that is not fully understood. Androgens from the adrenal gland are also responsible for hair distribution in both sexes.

HORMONES IN DEVELOPMENT AND REPRODUCTION IN FEMALES

At birth and throughout childhood cells around the egg (ova) produce a substance that inhibits its maturation. During childhood the ovaries remain essentially dormant largely as a result of a lack of hormonal stimulation. Although the ovaries are stimulated during fetal development by hormones released from the placenta (human chorionic gonadotropins), the level of those hormones becomes negligible within a few weeks after birth and does not increase until the start of puberty around the age of nine or ten. At that time estrogen production and secretion increase dramatically in females. Under hormonal stimulation the ovaries, fallopian tubes, uterus, and vagina all increase in size. The external genitalia, including the labia majora and minora, also grow to their mature size. The presence of estrogen also changes the lining of the vagina to make it more resilient and better able to resist trauma and infection. Estrogen also causes the development of breast tissue by increasing the ductile system and the deposition of fat.

At puberty the anterior pituitary gland begins secreting increased amounts of FSH and LH, causing the ovaries and the follicles (the egg and its surrounding tissue) to grow. This consequently stimulates the onset of monthly sexual cycles (menarche), which generally begin between the ages of eleven and sixteen.

The female sexual cycle is characterized by a rhythmic pattern of hormonal release that causes changes in the ovaries and sex organs and influences a woman's fertility. These cycles may last anywhere from twenty to forty-five days (averaging twenty-eight). The primary result of these monthly changes is to produce and release a mature egg (ovum) and prepare the lining of the uterus (the endometrium) for implantation of a fertilized egg.

In a twenty-eight-day cycle FSH and later LH increase slightly or moderately during the first few days of menstruation. This causes six to twelve ripe follicles (the sac of tissue that surrounds each egg) to increase in size. After a few days the follicle begins to produce a fluid with a high concentration of estrogen. Although this first stage of follicular development is the primarily the result of FSH stimulation, the subsequent acceleration in growth results from the increasing presence of estrogen, which makes the follicle even more sensitive to FSH stimulation. This combination of estrogen and FSH subsequently makes the follicle even more sensitive to LH stimulation. The increased influence of estrogen and LH causes the follicle to grow very rapidly. After a week or so (and before ovulation) one of the follicles grows much larger than the others, and the smaller ones then involute (degenerate), leaving only one follicle ready for ovulation.

About two days before ovulation there is a marked increase in the amount of LH secreted that peaks about sixteen hours before ovulation (on day fourteen). FSH also increases at that time, though not as markedly as LH does. The two hormones act together to cause the follicle to rapidly swell, rupture, and release the ovum into the abdomen. Around that time LH also causes the cells surrounding the ovum to begin producing progesterone and less estrogen.

In the hours after the egg is expelled from the follicle the remaining cells of the follicle change into lutein cells and convert what remains of the follicle to what is called the corpus luteum: glandular tissue that produces large quantities of progesterone and to a lesser degree estrogen. The corpus luteum also produces small quantities of the male hormones (including testosterone), but these hormones ultimately are converted to female hormones. The presence of progesterone and estrogen inhibits the anterior pituitary gland from secreting FSH and LH. As a result the corpus luteum continues to grow for about seven or eight days after ovulation, after which, if no pregnancy occurs, it begins to degenerate. Its involution leads to a subsequent decrease in progesterone and estrogen that stimulates the uterus to menstruate two days later.

HORMONES AND THE PLACENTA

The placenta (the nourishing sac that surrounds the growing fetus) produces large quantities of estrogen. In pregnancy the chorion (the layer of cells surrounding the developing embryo) produces and secretes hormone human chorionic gonadotropin, which causes the corpus leuteum to remain functional. The secretion of HCG reaches its peak about eight or nine weeks after fertilization. It then goes into decline and is maintained at that low level throughout the remainder of the pregnancy (most pregnancy kits test for the presence of HCG in the blood or urine). The placenta then begins to increase the secretion of estrogens and progesterones. By the end of the first trimester of pregnancy the placenta is a functional endocrine gland that secretes enough estrogen and progesterone to maintain the pregnancy until childbirth. After delivery, in the absence of this high concentration of estrogen and progesterone, prolactin (produced by the anterior pituitary) stimulates lactation (milk production). Suckling stimulates the posterior pituitary to release oxy-tocin (which stimulates the breasts cells to contract and aid in milk let-down) and the anterior pituitary to continue to secrete prolactin.

EFFECTS OF THE LOSS OF HORMONES

The loss of hormones may result from genetic abnormality, disease, treatment (such as chemotherapy or radiation), surgery (removal of the ovaries or testes), or aging. Decreased production of sex hormones may cause changes that can interfere with sexual behavior or physiology in both women and men. In women a marked loss of estrogen manifests with menopause-like symptoms characterized by cessation of menstrual periods, loss of fertility, "hot flashes," irritability, fatigue, atrophic vaginitis (thinning of the vaginal wall), and osteoporosis (loss of bone mass).

Men also may experience the loss of hormone testosterone as a result of disease, treatments, or aging (in what controversially is called andropause or "male menopause"). Symptoms may manifest as a loss of energy, muscle mass, physical agility, fertility, and sex drive as well as sexual dysfunction (impotence). In addition, the loss of testosterone is associated with an increased risk for cardiovascular disease. The loss of the hormone also may prevent male pattern baldness from progressing.

BIBLIOGRAPHY

Berne, Robert M., et al., eds. 1998. Physiology. 4th edition. St. Louis, MO: Mosby.

Guyton, Arthur C. 1991. Textbook of Medical Physiology. 8th edition. Philadelphia: Saunders.

Pfaff, Donald W.; Arthur P. Arnold; Anne M. Etgen, et al., eds. 2002. Hormones, Brain and Behavior. Vol. 1. Amsterdam and Boston: Academic Press.

                                            Diane Sue Saylor

III. HORMONAL DISEASES

The loss of hormones can cause physiological changes that may interfere with sexual characteristics or behavior. That loss may result from genetic abnormalities, disease, treatment (such as chemotherapy or radiation), surgery (removal of the ovaries or testes), or aging. In women a marked loss of estrogen manifests with menopause-like symptoms characterized by cessation of menstrual periods, loss of fertility, vasomotor symptoms ("hot flushes" or "hot flashes"), irritability, fatigue, atrophic vaginitis (thinning of the vaginal walls), and osteoporosis (loss of bone mass). Although estrogen replacement therapy may help with symptoms, it no longer is recommended except on a short-term basis because of the increased risk of cardiovascular disease and breast cancer. Other remedies include over-the-counter herbal preparations such as black cohosh, soy-based preparations, and selective serotonin reuptake inhibitors (SSRIs) and antidepressants such as Paxil. Because of its effect on bone density, estrogen sometimes is prescribed to treat osteoporosis, which is a serious health concern for postmenopausal women.

Tumors of the adrenal gland occasionally may cause the overproduction of androgens, resulting in masculinizing secondary sexual characteristics in women, including facial hair growth (hirsutism). Tumors in the embryonic tissue of the ovaries and adrenal hormone abnormalities also may lead to the production of large quantities of androgens. When this happens in genetically female fetuses, masculinizing occurs and may result in female pseudohermaphrodism characterized by the possession of ovaries and ambiguous genitalia.

Normal aging results in a loss of testosterone production in males (andropause). Symptoms may manifest as a loss of energy, muscle mass, physical agility, fertility, and sex drive as well as sexual dysfunction (impotence). In addition, the loss of testosterone is associated with an increased risk for cardiovascular disease. It also may prevent male pattern baldness from progressing. Treatment of hormone insufficiency may include testosterone injections, patches, or implants.

If the testes fail to function during fetal development (hypogonadism), there are insufficient androgens to stimulate the growth of male sexual organs, and normal female organs develop instead. If an adolescent boy loses his testes before puberty, he will not develop mature sexual characteristics: His voice will remain childlike, he will not go bald, and he will not develop the facial and body hair characteristic of an adult male. If a male loses his testes after puberty, some of the masculine secondary sexual characteristics, such as sex organ size and a deepened voice, are maintained for the most part (though slightly diminished), whereas others, such as hair distribution and musculature and bone mass, may decrease markedly. Adult men who experience a loss of testosterone may note decreased sexual desire and difficulty achieving an erection. As with aging, these symptoms may be treated by the administration of testosterone. Loss of testosterone production in the testes can cause the prostate to diminish in size. Further, certain cancers of the prostate can be stimulated by testosterone. These types of cancer may be treated by removal of the testes or the administration of estrogen.

Hypergonadism (overproduction of sex hormones) may occur with testicular tumors. When this occurs in young children, up to 100 times the normal amount of testosterone may be secreted, causing the bones to fuse at an early age, before full adult height has been reached. The increased quantity of testosterone also stimulates a premature and excessive development of the sex organs and secondary sexual characteristics. In adults hypergo-nadism often goes undiagnosed because sexual characteristics already have developed. Other tumors of the genital tissue may produce large amounts of luteinizing hormone (LH) or estrogen, causing symptoms that include gyne-comastia (overgrowth of the breasts).

see also Contraception: I. Overview; Genitals, Female; Genitals, Male.

BIBLIOGRAPHY

Berne, Robert M., et al., eds. 1998. Physiology. 4th edition. St. Louis, MO: Mosby.

Guyton, Arthur C. 1991. Textbook of Medical Physiology, 8th edition. Philadelphia: Saunders.

Speroff, Leon; Robert H. Glass; and Nathan G. Kase. 1994. Clinical Gynecologic Endocrinology and Infertility. 5th edition. Baltimore, MD: Williams & Wilkins.

                                             Diane Sue Saylor

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Hormones

Hormones

Hormones are biochemical messengers that regulate physiological events in living organisms. More than 100 hormones have been identified in humans. Hormones are secreted by endocrine (ductless) glands such as the hypothalamus, the pituitary gland, the pineal gland, the thyroid, the parathyroid, the thymus, the adrenals , the pancreas, the ovaries, and the testes. Hormones are secreted directly into the blood stream from where they travel to target tissues and modulate digestion, growth, maturation, reproduction, and homeostasis . The word hormone comes from the Greek word, hormon, to stir up, and indeed excitation is characteristic of the adrenaline and the sex hormones. Most hormones produce an effect on specific target tissues that are sited at some distance from the gland secreting the hormone. Although small plasma concentrations of most hormones are always present, surges in secretion trigger specific responses at one or more targets. Hormones do not fall into any one chemical category, but most are either protein molecules or steroid molecules. These biological managers keep the body systems functioning over the long term and help maintain health. The study of hormones is called endocrinology.

Mechanisms of action

Hormones elicit a response at their target tissue , target organ , or target cell type through receptors. Receptors are molecular complexes which specifically recognize another molecule-in this case, a particular hormone. When the hormone is bound by its receptor, the receptor is usually altered in some way that it sends a secondary message through the cell to do something in response. Hormones that are proteins , or peptides (smaller strings of amino acids), usually bind to a receptor in the cell's outer surface and use a second messenger to relay their action. Steroid hormones such as cortisol, testosterone, and estrogen bind to receptors inside cells. Steroids are small enough to and chemically capable of passing through the cell's outer membrane . Inside the cell, these hormones bind their receptors and often enter the nucleus to elicit a response. These receptors bind DNA to regulate cellular events by controlling gene activity.

Most hormones are released into the bloodstream by a single gland. Testosterone is an exception, because it is secreted by both the adrenal glands and by the testes. Plasma concentrations of all hormones are assessed at some site which has receptors binding that hormone. The site keeps track of when the hormone level is low or high. The major area which records this information is the hypothalamus. A number of hormones are secreted by the hypothalamus which stimulate or inhibit additional secretion of other hormones at other sites. The hormones are part of a negative or positive feedback loop.

Most hormones work through a negative feedback loop. As an example, when the hypothalamus detects high levels of a hormone, it reacts to inhibit further production. And when low levels of a hormone are detected, the hypothalamus reacts to stimulate hormone production or secretion. Estrogen, however, is part of a positive feedback loop. Each month, the Graafian follicle in the ovary releases estrogen into the bloodstream as the egg develops in ever increasing amounts. When estrogen levels rise to a certain point, the pituitary secretes luteinizing hormone (LH) which triggers the egg's release of the egg into the oviduct.

Not all hormones are readily soluble in blood (their main transport medium) and require a transport molecule that will increase their solubility and shuttle them around until they get to their destination. Steroid hormones, in particular, tend to be less soluble. In addition, some very small peptides require a carrier protein to deliver them safely to their destination, because these small peptides could be swept into the wrong location where they would not elicit the desired response. Carrier proteins in the blood include albumin and prealbumin. There are also specific carrier proteins for cortisol, thyroxin, and the steroid sex hormones.


Major hormones

The concentrations of several important biological building blocks such as amino acids are regulated by more than one hormones. For example, both calcitonin and parathyroid hormone (PTH) influence blood calcium levels directly, and other hormones affect calcium levels indirectly via other pathways.


The hypothalamus

Hormones secreted by the hypothalamus modulate other hormones. The major hormones secreted by the hypothalamus are corticotrophin releasing hormone (CRH), thyroid stimulating hormone releasing hormone (TRH), follicle stimulating hormone releasing hormone (FSHRH), luteinizing hormone releasing hormone (LRH), and growth hormone releasing hormone (GHRH). CRH targets the adrenal glands. It triggers the adrenals to release adrenocorticotropic hormone (ACTH). ACTH functions to synthesize and release corticosteroids. TRH targets the thyroid where it functions to synthesize and release the thyroid hormones T3 and T4. FSH targets the ovaries and the testes where it enables the maturation of the ovum and of spermatozoa. LRH also targets the ovaries and the testes, and its receptors are in cells which promote ovulation and increase progesterone synthesis and release. GHRH targets the anterior pituitary to release growth hormones to most body tissues, increase protein synthesis, and increase blood glucose. Hence, the hypothalamus plays a first domino role in these cascades of events.

The hypothalamus also secretes some other important hormones such as prolactin inhibiting hormone (PIH), prolactin releasing hormone (PRH), and melanocyte inhibiting hormone (MIH). PIH targets the anterior pituitary to inhibit milk production at the mammary gland, and PRH has the opposite effect. MIH targets skin pigment cells (melanocytes) to regulate pigmentation.


The pituitary gland

The pituitary has long been called the master gland because of the vast extent of its activity. It lies deep in the brain just behind the nose. The pituitary is divided into anterior and posterior regions with the anterior portion comprising about 75% of the total gland. The posterior region secretes the peptide hormones vasopressin, also called anti-diuretic hormone (ADH), and oxytocin. Both are synthesized in the hypothalamus and moved to the posterior pituitary prior to secretion. ADH targets the collecting tubules of the kidneys, increasing their permeability to water . ADH causes the kidneys to retain water. Lack of ADH leads to a condition called diabetes insipidus characterized by excessive urination. Oxytocin targets the uterus and the mammary glands in the breasts. Oxytocin begins labor prior to birth and also functions in the ejection of milk. The drug, pitocin, is a synthetic form of oxytocin and is used medically to induce labor.

The anterior pituitary (AP) secretes a number of hormones. The cells of the AP are classified into five types based on what they secrete. These cells are somatotrophs, corticotrophins, thyrotrophs, lactotrophs, and gonadotrophs. Respectively, they secrete growth hormone (GH), ACTH, TSH, prolactin, and LH and FSH. Each of these hormones is either a polypeptide or a glycoprotein. GH controls cellular growth, protein synthesis, and elevation of blood glucose concentration . ACTH controls secretion of some hormones by the adrenal cortex (mainly cortisol). TSH controls thyroid hormone secretion in the thyroid. In males, prolactin enhances testosterone production; in females, it initiates and maintains LH to promote milk secretion from the mammary glands. In females, FSH initiates ova development and induces ovarian estrogen secretion. In males, FSH stimulates sperm production in the testes. LH stimulates ovulation and formation of the corpus luteum which produces progesterone. In males, LH stimulates interstitial cells to produce testosterone. Each AP hormone is secreted in response to a hypothalamic releasing hormone.


The thyroid gland

The thyroid lies under the larynx and synthesizes two hormones, thyroxine and tri-iodothyronine. This gland takes up iodine from the blood and has the highest iodine level in the body. The iodine is incorporated into the thyroid hormones. Thyroxine has four iodine atoms and is called T4. Tri-iodothyronine has three iodine atoms and is called T3. Both T3 and T4 function to increase the metabolic rate of several cells and tissues. The brain, testes, lungs, and spleen are not affected by thyroid hormones, however. T3 and T4 indirectly increase blood glucose levels as well as the insulin-promoted uptake of glucose by fat cells. Their release is modulated by TSH-RH from the hypothalamus. TSH secretion increases in cold infants. When temperature drops, a metabolic increase is triggered by TSH. Chronic stress seems to reduce TSH secretion which, in turn, decreases T3 and T4 output.

Depressed T3 and T4 production is the trademark of hypothyroidism. If it occurs in young children, then this decreased activity can cause physical and mental retardation. In adults, it creates sluggishness—mentally and physically—and is characterized further by weight gain, poor hair growth, and a swollen neck. Excessive T3 and T4 cause sweating, nervousness, weight loss, and fatigue. The thyroid also secretes calcitonin which serves to reduce blood calcium levels. Calcitonin's role is particularly significant in children whose bones are still forming.


The parathyroid glands

The parathyroid glands are attached to the bottom of the thyroid gland. They secrete the polypeptide parathyroid hormone (PTH) which plays a crucial role in monitoring blood calcium and phosphate levels. About 99% of the body's calcium is in the bones, and 85% of the magnesium is also found in bone. Low blood levels of calcium stimulate PTH release into the bloodstream in two steps. Initially, calcium is released from the fluid around bone cells. And later, calcium can be drawn from bone itself. Although, only about 1% of bone calcium is readily exchangeable. PTH can also increase the absorption of calcium in the intestines by stimulating the kidneys to produce a vitamin D-like substance which facilitates this action. High blood calcium levels will inhibit PTH action, and magnesium (which is chemically similar to calcium) shows a similar effect.

Calcium is a critical element for the human body. Even though the majority of calcium is in bone, it is also used by muscles, including cardiac muscle for contractions, and by nerves in the release of neurotransmitters. Calcium is a powerful messenger in the immune response of inflammation and blood clotting. Both PTH and calcitonin regulate calcium levels in the kidneys, the gut, bone, and blood. Whereas calcitonin is released in conditions of high blood calcium levels, PTH is released when calcium levels fall in the blood. Comparing the two, PTH causes an increase in calcium absorption in the kidneys, absorption in the intestine, release from bone, and levels in the blood. In addition, PTH decreases kidney phosphate absorption. Calcitonin has the opposite effect on each of these variables. PTH is thought to be the major calcium modulator in adults.

PTH deficiency can be due to autoimmune diseases or to inherited parathyroid gland problems. Low PTH capabilities cause depressed blood calcium levels and neuromuscular problems. Very low PTH can lead to tetany or muscle spasms. Excess PTH can lead to weakened bones because it causes too much calcium to be drawn from the bones and to be excreted in the urine. Abnormalities of bone mineral deposits can lead to a number of conditions including osteoporosis and rickets. Osteoporosis can be due to dietary insufficiencies of calcium, phosphate, or vitamin C (which has an important role in formation of the bone matrix). The end result is a loss of bone mass. Rickets is usually caused by a vitamin D deficiency and results in lower rates of bone matrix formation in children. These examples show how important a balanced nutritious diet is for healthy development.


The adrenal glands

The two adrenal glands, one on top of each kidney, each have two distinct regions. The outer region (the medulla) produces adrenaline and noradrenaline and is under the control of the sympathetic nervous system . The inner region (the cortex) produces a number of steroid hormones. The cortical steroid hormones include mineralocorticoids (mainly aldosterone), glucocorticoids (mainly cortisol), and gonadocorticoids. These steroids are derived from cholesterol . Although cholesterol receives a lot of bad press, some of it is necessary. Steroid hormones act by regulating gene expression, hence, their presence controls the production of numerous factors with multiple roles. Aldosterone and cortisol are the major human steroids in the cortex. However, testosterone and estrogen are secreted by adults (both male and female) at very low levels.

Aldosterone plays an important role in regulating body fluids. It increases blood levels of sodium and water and lowers blood potassium levels. Low blood sodium levels trigger aldosterone secretion via the renin-angiotensin pathway. Renin is produced by the kidney, and angiotensin originates in the liver. High blood potassium levels also trigger aldosterone release. ACTH has a minor promoting effect on aldosterone. Aldosterone targets the kidney where it promotes sodium uptake and potassium excretion. Since sodium ions influence water retention, the result is a net increase in body fluid volume .

Blood cortisol levels fluctuate dramatically throughout the day and peak around 8 a.m. Presumably, this early peak enables humans to face the varied daily stressors they encounter. Cortisol secretion is stimulated by physical trauma, cold, burns, heavy exercise , and anxiety . Cortisol targets the liver, skeletal muscle, and adi-pose tissue. Its overall effect is to provide amino acids and glucose to meet synthesis and energy requirements for normal metabolism and during periods of stress. Because of its anti-inflammatory action, it is used clinically to reduce swelling. Excessive cortisol secretion leads to Cushing syndrome which is characterized by weak bones, obesity , and a tendency to bruise. Cortisol deficiency can lead to Addison disease which has the symptoms of fatigue, low blood sodium levels, low blood pressure , and excess skin pigmentation.

The adrenal medullary hormones are epinephrine (adrenaline) and nor-epinephrine (nor-adrenaline). Both of these hormones serve to supplement and prolong the fight or flight response initiated in the nervous system. This response includes the neural effects of increased heart rate, peripheral blood vessel constriction, sweating, spleen contraction, glycogen conversion to glucose, dilation of bronchial tubes, decreased digestive activity, and lowered urine output.

The condition of stress presents a model for reviewing one way that multiple systems and hormones interact. During stress, the nervous, endocrine, digestive, urinary, respiratory, circulatory, and immune response are all tied together. For example, the hypothalamus sends nervous impulses to the spinal cord to stimulate the fight or flight response and releases CRH which promotes ACTH secretion by the pituitary. ACTH, in turn, triggers interleukins to respond which promote immune cell functions. ACTH also stimulates cortisol release at the adrenal cortex which helps buffer the person against stress. As part of a negative feedback loop, ACTH and cortisol receptors on the hypothalamus assess when sufficient levels of these hormones are present and then inhibit their further release. De-stressing occurs over a period of time after the stressor is gone. The systems eventually return to normal.


The pancreas

The pancreas folds under the stomach, secretes the hormones insulin , glucagon, and somatostatin. About 70% of the pancreatic hormone-secreting cells are called beta cells and secrete insulin; another 22%, or so, are called alpha cells and secrete glucagon. The remaining gamma cells secrete somatostatin, also known as growth hormone inhibiting hormone (GHIH). The alpha, beta, and gamma cells comprise the islets of Langerhans which are scattered throughout the pancreas.

Insulin and glucagon have reciprocal roles. Insulin promotes the storage of glucose, fatty acids , and amino acids, whereas, glucagon stimulates mobilization of these constituents from storage into the blood. Both are relatively short polypeptides. Insulin release is triggered by high blood glucose levels. It lowers blood sugar levels by binding a cell surface receptor and accelerating glucose transport into the cell where glucose is converted into glycogen. Insulin also inhibits the release of glucose by the liver in order to keep blood levels down. Increased blood levels of GH and ACTH also stimulate insulin secretion. Not all cells require insulin to store glucose, however. Brain, liver, kidney, intestinal, epithelium, and the pancreatic islets can take up glucose independently of insulin. Insulin excess can cause hypoglycemia leading to convulsions or coma , and insufficient levels of insulin can cause diabetes mellitus which can be fatal if left untreated. Diabetes mellitus is the most common endocrine disorder.

Glucagon secretion is stimulated by decreased blood glucose levels, infection , cortisol, exercise, and large protein meals. GHIH, glucose, and insulin inhibit its secretion. Protein taken in through the digestive tract has more of a stimulatory effect on glucagon than does injected protein. Glucagon stimulates glycogen breakdown in the liver, inhibits glycogen synthesis, and facilitates glucose release into the blood. Excess glucagon can result from tumors of the pancreatic alpha cells; and a mild diabetes seems to result. Some cases of uncontrolled diabetes are also characterized by high glucagon levels suggesting that low blood insulin levels are not always the only cause in some diabetes cases.

It was the study of glucagon and its action by Sutherland in 1961 that led to the concept of the second messenger system. Glucagon activates the intracellular molecule cyclic AMP, cAMP. Since this discovery, a number of other molecules have been found which modulate cellular activity via this second messenger.


The female reproductive organs

The female reproductive hormones arise from the hypothalamus, the anterior pituitary, and the ovaries. Although detectable amounts of the steroid hormone estrogen are present during fetal development, at puberty estrogen levels rise to initiate secondary sexual characteristics. Gonadotropin releasing hormone (GRH) is released by the hypothalamus to stimulate pituitary release of LH and FSH. LH and FSH propagate egg development in the ovaries. Eggs (ova) exist at various stages of development, and the maturation of one ovum takes about 28 days and is called the ovarian or menstrual cycle . The ova are contained within follicles which are support organs for ova maturation. About 450 of a female's 150,000 germ cells mature to leave the ovary. The hormones secreted by the ovary include estrogen, progesterone, and small amounts of testosterone.

As an ovum matures, rising estrogen levels stimulate additional LH and FSH release from the pituitary. Prior to ovulation, estrogen levels drop, and LH and FSH surge to cause the ovum to be released into the fallopian tube. The cells of the burst follicle begin to secrete progesterone and some estrogen. These hormones trigger thickening of the uterine lining, the endometrium, to prepare it for implantation should fertilization occur. The high progesterone and estrogen levels prevent LH and FSH from further secretion-thus hindering another ovum from developing. If fertilization does not occur, eight days after ovulation the endometrium deteriorates resulting in menstruation. The falling estrogen and progesterone levels which follow trigger LH and FSH, starting the cycle all over again.

Although estrogen and progesterone have major roles in the menstrual cycle, these hormones have receptors on a number of other body tissues. Estrogen has a protective effect on bone loss which can lead to osteoporosis. And progesterone, which is a competitor for androgen sites, blocks actions that would result from testosterone activation. Estrogen receptors have even been found in the forebrain indicating a role in female neuronal function or development.

Hormones related to pregnancy include human chorionic gonadotrophin (HCG), estrogen, human chorionic somatomammotrophin (HCS), and relaxin. HCG is released by the early embryo to signal implantation. Estrogen and HCS are secreted by the placenta. And relaxin is secreted by the ovaries as birth nears to relax the pelvic area in preparation for labor.


The male reproductive organs

Male reproductive hormones come from the hypothalamus, the anterior pituitary, and the testes. As in females, GRH is released from the hypothalamus which stimulates LH and FSH release from the pituitary. In males, LH and FSH facilitate spermatogenesis. The steroid hormone testosterone is secreted from the testes and can be detected in early embryonic development up until shortly after birth. Testosterone levels are quite low until puberty. At puberty, rising levels of testosterone stimulate male reproductive development including secondary characteristics.

LH stimulates testosterone release from the testes. FSH promotes early spermatogenesis, whereas testosterone is required to complete spermatogenic maturation to facilitate fertilization. In addition to testosterone, LH, and FSH, the male also secretes prostaglandins. These substances promote uterine contractions which help propel sperm towards an egg in the fallopian tubes during sexual intercourse. Prostaglandins are produced in the seminal vesicles, and are not classified as hormones by all authorities.

See also Biological rhythms; Cell; Endocrine system; Exocrine glands; Glands; Growth hormones; Reproductive system.


Resources

books

Burnstein, K. L. Steroid Hormones and Cell Cycle Regulation. Boston: Kluwer Academic Publishers, 2002.

Engelking, L. R. Metabolic and Endocrine Physiology. Jackson, WY:, Teton NewMedia, 2000.

Goffin, V., P. A. Kelly. Hormone Signaling. Boston: Kluwer Academic Publishers, 2002.

Griffin, J. E., and S. R. Ojeda Textbook of Endocrine Physiology. New York: Oxford University Press, 2000.

Kacsoh, B. Endocrine Physiology. New York: McGraw-Hill Health Professions Division, 2000.


Louise Dickerson

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Amino acid

—An organic compound whose molecules contain both an amino group (-NH2) and a carboxyl group (-COOH). One of the building blocks of a protein.

Androgen

—Any hormone with testosterone-like activity (i.e. it increases male characteristics).

Homeostasis

—A condition of chemical and physical equilibrium in the human body.

Plasma

—The non-cellular, fluid portion of blood in which the concentration of most molecules is measured.

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Hormones

Hormones


Hormones are chemical messengers found in both animals and plants. In animals, hormones are produced by glands and travel through the blood to certain target tissues. There they act as chemical regulators. Hormones influence reproduction, growth, and overall bodily balance, among other things.

Hormones are important to both plants and animals, but especially to animals. Hormones regulate key bodily functions like body growth, sexual maturity, reproduction, and the maintenance of a stable, or balanced, internal environment. Some hormones have a temporary effect, such as those that regulate the body's blood sugar level. Others cause permanent changes, such as those that make a person grow tall and mature sexually. Still others are present only in certain situations, such as those that prepare a body for stressful situations. Whatever their effect, hormones help an organism to adapt to its environment in the best manner possible. The word hormone comes from the Greek hormaein meaning to excite or to set into motion, and this describes what hormones do—they have a stimulating effect.

In vertebrate animals (animals with a backbone), hormones are produced by certain glands, tissues, or organs. They travel through the circulatory system (a network that carries blood throughout an animal's body) to target cells. Hormones do not produce an effect until they reach these specifically receptive cells. The target cells are programmed to react when stimulated by a certain hormone. Only the target cells in the target organ are able to produce the desired effect, since they have receptors that recognize and bind to the hormone.

THE ENDOCRINE SYSTEM

It is estimated that vertebrates have at least fifty different hormones, and many are produced by what is called the endocrine system. Some of the major glands in the human endocrine system are the pituitary gland and the pineal gland at the base of the brain; the thyroid and parathyroid in the throat; and the adrenal, gonads, thymus, and pancreas in the trunk or lower half of the body. Each of these endocrine glands releases its own particular hormone into the bloodstream and each produces the desired effect when it reaches the appropriate target cells. Thus, some hormones

stimulate the growth of muscle and bone, others begin the secretion of milk, and still others influence blood pressure. All are usually produced in tiny quantities, yet all have profound and major effects on the body.

HORMONES IN HUMANS

For example, female sex hormones, like the group called estrogens, are produced by the pituitary gland as well as the ovaries. These hormones produce the dramatic physical changes that take place when a young girl starts to become a young woman. The estrogens trigger the development of what are called secondary sexual characteristics, like breasts. Later, if a woman becomes pregnant, these and other hormones prepare her body to carry and nourish a developing fetus. The male sex hormone, testosterone, is made by the testes and produces the typical male secondary sex characteristics during puberty.

Another well-known hormone is adrenaline. This produces what is known as the "fight-or-flight" response. When an individual senses danger, the body automatically enters a state of readiness to either fight for survival or to take measures to avoid a conflict. This powerful hormone works with great suddenness and a person can actually feel its effects. The heart rate and blood pressure quicken, the skin goes pale, the body's blood sugar level rises, and a person's strength increases. All of these and other physiological reactions take place immediately and without the person's conscious will. These reactions give people a better chance to act immediately and possibly survive a threat.

In humans and animals, hormones are made of either proteins or steroids (which are a type of lipid or organic compound that includes fats, oils, and waxes). The adrenal gland and the gonads (male testes and female ovaries) produce steroid hormones, while the rest of the endocrine system makes hormones that are protein-based and, therefore, are made out of amino acids (the building blocks of proteins). Abnormally functioning endocrine glands can result in too much or too little hormones. For example, a lack of human growth hormone from the pituitary gland can result in dwarfism, while too much can produce giants who suffer from a condition known as acromegaly.

HORMONES IN OTHER ORGANISMS

While other living things also have hormones, their hormones are nowhere near as dominating an influence as they are for animals. Other organisms do not have as elaborate a system of transport and reception as do vertebrates. However, hormones are still very important to invertebrates (animals without a backbone). Invertebrates use hormones mainly in their growth and development. For example, insects that molt, or periodically shed, their skin produce a hormone that allows this to happen at the right time. Metamorphosis (the complete bodily change that takes place in an insect, such as when a caterpillar changes into a butterfly) is controlled by hormones. When an octopus changes its color during stress, it is a hormone that causes this dramatic reaction.

ERNEST HENRY STARLING

English physiologist Ernest Starling (1866–1927) helped create endocrinology, a major branch of medicine and physiology that studies the glands of the body. Starling not only discovered the digestive hormone secretin, but he also suggested the name "hormone" to describe the body's chemical messengers.

Ernest Starling was born in London, England. His father worked in Bombay, India, for the British government and was able to come home only once every three years. The young Starling was educated at King's College School in London and then enrolled at Guy's Hospital Medical College in London. While there, he took advantage of the opportunity to do research in Heidelberg, Germany, and study with the eminent German physiologist, Wilhelm Kuhne (1837–1900). After obtaining his medical degree in 1889, he was appointed demonstrator of physiology at Guy's, and he eventually became head of the department of physiology there. By the time he left there in 1899 to become Professor of Physiology at University College, he had made a name for himself studying the conditions that cause fluids to leave blood vessels and enter the tissues. In fact, in 1896 he demonstrated a phenomenon that came to be known as "Starling equilibrium."

Starling is best known, however, for his work with the English physiologist William Maddock Bayliss (1860–1924), who became his brother-in-law in 1893 when he married Starling's sister. Together they began a study of the secretion of digestive juices by the pancreas (a gland). The normal pancreas releases several different juices into the duodenum (the top part of the small intestine) to assist digestion. After they had cut all the nerves to the pancreas, they found that the organ continued to release its juices. This proved that it was not under nervous control (that is, not controlled directly by the brain), and so they concluded in 1902 that it must have received a chemical rather than an electrical message. This meant that the message must have been sent to the pancreas through the blood when food entered the duodenum.

They soon found that the small intestine secretes a substance, or a chemical messenger, into the blood that they named "secretin." Further research showed that the secretin was released under the influence of stomach acid. This was the first time that a certain chemical had been proven to act as a stimulus for an organ that was located somewhere else in the body. Starling and Bayliss eventually came to call any chemical that transmits a message from one part of the body to another part a "hormone." This word was taken from the Greek root meaning "to excite." Although hormones had actually been known before the discovery of secretin in 1902, it was Starling who first clearly defined the concept in 1905 and who detailed the role that such substances play in the body. It was thought that Starling and Bayliss were strong candidates for a Nobel Prize, but World War I (1914–18) intervened, and no awards were given for those years. As for recognition from his own country, Starling had been such an outspoken critic of the way his country had managed the war effort that he was given no awards in his lifetime.

For plants, hormones allow them to react to the changing conditions of their environment. Some hormones promote cell division, others stimulate or slow growth, and others cause a plant's fruit to ripen. Plants do not have specialized structures for hormones as animals do. In fact, a plant can even be affected by the hormone of its neighbor. This sometimes occurs when a plant releases the ripening hormone called ethylene into the atmosphere. Thus, a fruit like an apple continues to produce this ripening hormone even after it is picked, and will therefore speed up the ripening of any other fruit nearby.

[See alsoEndocrine System; Reproduction System ]

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Hormones

Hormones

The regulation of the coordinated effort between the various organs, cells, and systems within the human body is a remarkable communications structure. The messengers that transport the signals for all manner of activity between the various centers of the body are the chemicals known as hormones. Hormones are also known by their biological classification, endocrines, which are defined as the chemicals that are carried by either blood or by the tissue structures of the body to a determined place, to initiate a predetermined action. These structures are in contrast to the exocrines, which include the saliva and the tear duct products of the body; the exocrine structures do not direct their product within the body, but expel chemicals from the body.

Hormones are produced by the small organs known as glands. Glands produce the chemicals used to signal desired changes in cellular or system performance through the receipt of materials from the bloodstream and then fashioning the particular hormone specific to the gland. The important glandular structures of the body are organized into a hierarchy. The impetus for most hormonal production begins with the signals that are transmitted by the hypothalamus, the portion of the brain that is responsible for the coordination of many forms of organ and system responses.

When the hypothalamus determines that the body must take action of some kind in response to an external stimulus, such as an athlete preparing to compete in a soccer game, a series of chemical signals are transmitted from the hypothalamus to the organ known as the "master gland," the pituitary. The pituitary is responsible for both the ongoing maintenance and regulation of several critical functions, including the production of the human growth hormone (HGH). The pituitary also regulates the function of the glands that ultimately provide the distinct chemical responses directed by the hypo-thalamus, the thyroid gland and the adrenal glands.

Insulin and glucagon are hormones produced in the pancreas, the small organ located below the stomach. These chemicals are substances that operate in contrast; insulin is the regulatory agent essential to the maintenance of an optimal glucose balance, signaling the liver to store more glucose when the blood sugar level is too high, whereas glucagon conveys the opposite signal, to release greater amounts of glucose into the bloodstream, when the levels are too low. In an ideal state, the influences of insulin and glucagon upon blood glucose levels are balanced.

The female ovaries and the male testes also have glands that are related to human reproduction. Each of these organs contain endocrine cells that permit the organ to produce the hormones necessary to produce eggs in the case of the female, and sperm cells in the case of the male.

The hormones produced by the glands are one of two general types: protein hormones (which include related substances known as peptides and modified amino acids), and steroid hormones, composed of various types of ringed, carbon-based molecules. The distinct structure of each determines how these hormones are physically transported through the bloodstream and tissues of the body. While insulin and glucagon operate in reference to the present level of one another (antagonistically), many important hormones are secreted in response to the presence or absence of the substance to which they are directed.

Other important hormones and their glandular production sites include the pituitary, pineal, thyroid, and adrenal glands, as well as erythropoietin (EPO). The pituitary gland (located in the brain) functions as the master gland, with a distinct regulatory function in relation to the thyroid and adrenal glands, as well as the production of the human growth hormone, the substance that regulates both the rate and timing of growth patterns within the body. The pineal gland produces melatonin, a hormone that is important in the regulation of the production of other hormones. The thyroid gland (found in the neck) regulates a variety of related metabolic and energy-generating functions, through the production of a number of specific hormones. The adrenal glands (one located at each kidney) produce adrenaline, the "fight or flight response" hormone, which influences heart rate, respiration, and other organ functions when adrenaline is released, in response to circumstances of excitement or threat; the adrenal glands also produce the hormonal direction to the kidneys to conserve water and sodium and to maintain the balance between those substances in the body when the glands are signaled that there may be imminent dehydration. Erythropoietin (EPO) is produced in the kidneys. This hormone will trigger increased production of erythrocytes (red blood cells), whose increased numbers within the bloodstream provide the body with a greater capacity to transport oxygen, to counter the effects of high altitudes, and to correspondingly reduce oxygen levels upon the aerobic and anaerobic energy systems.

Hormonal imbalances sometimes occur within the body. Such imbalances may be caused by external factors, such as improper diet, or through natural processes such as female menopause.

see also Cardiovascular system; High altitude effects on sport performance; Hormonal response to exercise.

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