Endocrine System

Endocrine System

The endocrine system is part of the regulatory system in animals and helps maintain the internal balance of the body. Both vertebrates and invertebrates have endocrine systems. The endocrine system regulates many functions of the body, including growth and metabolism, water balance, sugar and calcium balance in the bloodstream, and several functions related to sexual maturity and reproduction. Two major functions under endocrine control in invertebrates are the shedding of the exoskeleton for growth, called molting, and metamorphosis, functions that do not occur in vertebrates.

The endocrine system is not as fast to respond to stimuli as is the nervous system (the other major regulatory system in animals), which can respond in less than a second. The endocrine system can respond within minutes, and the effects usually last longer than the effects of the nervous system.

The endocrine system is made up of organs that produce chemical messengers called hormones. Hormones are released directly into the bloodstream in vertebrates and the haemolymph in invertebrates. Hormones circulate with the blood, so they are everywhere in the body.

Only certain cells, however, are capable of responding to these chemical messengers. These are target cells, which have special receptors for different kinds of hormones. Every chemical messenger has a unique shape. The target cell has a receptor that corresponds to the shape of the messenger. Most receptors are outside of a cell, embedded in the cell membrane.

When a messenger binds to the target, a different messenger is released inside of the cell. This second signal inside the cell is called a secondary messenger. This secondary messenger then triggers other changes inside the cell, such as the release of a substance. Other target cells have receptors on the inside of the cells. Specifically, some hormones can go inside of the cell and bind to a receptor that turns on and off DNA transcription of specific genes.

Endocrine Organs and Effects

The endocrine system works through the same process in vertebrates and invertebrates, although the organs and chemical messengers involved differ. In invertebrates, the nervous system has modified cells that secrete most types of hormones. The hormones released from within the nervous system regulate the other endocrine organs in invertebrates.

These other organs include the corpora cardiaca, the prothoracic glands, and the corpora allata. The corpora cardiaca are located next to the brain and secrete hormones that control the prothoracic glands. The prothoracic glands are located behind the brain and secrete ecdysone, which stimulates and controls molting, as well as other hormones involved in the molting process. The corpora allata is located near the digestive system and secretes juvenile hormone. Juvenile hormone is involved in growth, metamorphosis, and reproduction. The gonads (ovaries and testes) also secrete hormones in the invertebrates and are involved in reproduction.

Vertebrates have more endocrine organs than invertebrates. The hypothalamus, pituitary gland, and pineal gland are located in the brain. The hypothalamus controls the pituitary gland. The pituitary gland controls water regulation and endocrine production of the gonads, and stimulates growth as well. The pineal gland controls biological rhythms such as sleep by producing melatonin.

All other endocrine organs are located in the body cavity. The pancreas controls blood sugar levels by secreting two hormones that have the opposing functions of raising and lowering blood sugar levels. The thyroid and parathyroid control calcium levels in a manner similar to the pancreas: one hormone raises calcium and another lowers calcium levels. The thyroid also controls metabolism. The adrenal glands are located above the kidney. They are involved in both long-term and short-term stress responses. The thymus is involved in immune responses. The gonads are involved in many functions.

The gonads consist of the ovaries and testes and in vertebrates control development and growth in addition to regulating reproduction. The gonads secrete steroid hormones. Steroids are one of the chemical messengers that have receptors inside of target cells and most cells have steroid receptors, so that steroids affect the entire body.

The gonads produce three classes of steroid hormones: androgens that include testosterone, estrogens, and progestins. Both testes and ovaries produce all three steroid types, but in different proportions. In humans, steroids determine the sex of a fetus during development. If androgens are present at high levels during fetal development, then the fetus develops as a male. If androgens are not present at high levels, then the fetus develops as a female.

Steroids are also responsible for sexual maturation and the development of secondary sex characteristics during puberty in humans. Secondary sex characteristics in males caused by high levels of androgens include changing patterns in hair growth such as baldness and facial hair growth and deepening of the voice. Estrogen in females cause secondary sex characteristics such as the development of breasts. Progestins in females cause reproductive cycles and menstruation.

Supplemental Hormones for Humans

Humans sometimes take hormones by pills or injections to alter or supplement the body's own production of hormones. The best example of necessary hormone supplements is insulin replacement for diabetes mellitus. The pancreas secretes insulin, which lowers blood sugar levels, and glucagon, which raises blood sugar levels. When someone is diabetic, the body does not produce enough insulin and blood sugar remains at too high a level for normal water and metabolic functions.

Type I diabetes mellitus starts during childhood and is an autoimmune disease. Someone with Type I diabetes mellitus must take injections of insulin to control blood sugar levels. The insulin is either extracted from the organs of other animals or is produced by bioengineering bacteria to produce insulin. Those having Type II diabetes mellitus are often over forty years old and can control blood sugar levels with special diets and exercise.

Another common form of hormones taken by humans is steroids. A practice that is neither legal nor safe is that of individuals, usually males, taking steroids to increase muscle growth. Athletes of all types do this, not just bodybuilders. Androgens facilitate the acquisition of muscle mass, which is why men are more muscular than are women. However, taking supplemental androgens will cause the body to shut down its own production of androgens and interfere with the body's reproductive functions.

Common side effects of taking androgens include shrinking of the testes, impotence, the development of female secondary sex characters such as breasts, and a serious risk of heart attack. Additionally, sources for these androgens are usually other animals such as horses and illegal androgens are often impure, containing antibodies from the source animals. These antibodies can cause severe immune responses in humans and can even be fatal.

Birth control pills are another common form of steroids taken by humans. Birth control pills contain man-made estrogens and progestins. Birth control pills prevent ovulation, the development and release of an egg by the female, by disrupting the normal cycle of hormones that comprise the female menstrual cycle.

Environmental estrogens are chemicals that are thought to function as chemical messengers in animals. Examples of environmental estrogens include plastics and by-products of manufacturing. It is not completely understood at this point whether or not environmental estrogens can affect animals, and if so, to what degree.

Laura A. Higgins

Bibliography

Crews, David. "Animal Sexuality." Scientific American 270 (January 1994):108-115.

Johnson, George B. Biology: Visualizing Life. New York: Holt, Rinehart and Winston Inc., 1998.

McLachlan, John A., and Steven F. Arnold. "Environmental Estrogens." AmericanScientist 84 (September-October 1996):452-461.

Steinman, L. "Autoimmune Disease." Scientific American 269 (September 1993): 106-114.

Tjian, R. "Molecular Machines That Control Genes." Scientific American 272 (February 1995):54-61.

Haemolymph is the "blood" of invertebrates. Similar to mammalian blood, it carries nutrients to cells and waste away from cells. Haemolymph does not carry oxygen to cells like mammalian blood.

Hemolymph functions the same way as haemolymph, but works for insects instead of invertebrates.