pituitary gland

pituitary gland This gland, also termed the hypophysis cerebri, lies in a bony cavity, the sella turcica, so called because it was thought to resemble a Turkish saddle. It lies under the part of the brain known as the hypothalamus (whose location gives rise to its name, derived from the Greek, hypo meaning under and phyen to grow). It is connected to the hypothalamus by the pituitary stalk and in man is divided into two lobes, the anterior and the posterior, which develop in the embryo from completely different types of cell. The anterior lobe arises from below — from the same source as the mouth — and is made up of hormone-producing cells; the posterior lobe is developmentally a downward extension of the brain, and contains the endings of nerve fibres that arise from nerve cell bodies in one of two groups of cells (‘nuclei’) in the hypothalamus.

The existence of the pituitary gland was known before the time of Aristotle (384–22 bc), but it was only in the twentieth century that its true function was identified. Galen, the Greek physician and dogmatic teacher whose writing dominated Byzantine, Arabic, and medieval mdicine for a millennium, thought the pituita, one of four humours, passed from the brain to the nasal cavity. Vesalius (1514–64), a Belgian anatomist, was of a similar opinion, believing that waste material produced in the formation of the vital spirit was drained from the brain via the pituitary gland. This view was challenged in the seventeenth century and debate about its function continued through the eighteenth and into the nineteenth century. It was only at the end of the nineteenth century, when clinical disorders were recognized as being associated with pituitary tumours, that its real function as an endocrine organ was established.

The anterior pituitary

contains five different types of cell, each of which produce one particular hormone, with the exception of the ‘gonadotrophs’ which produce two: namely luteinizing hormone (LH) and follicular stimulating hormone (FSH). All the hormones are peptide or protein in nature, varying in size from 39 amino acids (ACTH) to 204 amino acids (LH and FSH). The hormones fall into two groups: the first contains the four trophic hormones (from the Greek for nourishment), which control other endocrine glands; the second contains prolactin and growth hormone, which have more widespread effects in the body.

The trophic hormones

act to stimulate secretion of hormone from the target gland and to maintain its function and, if present in high concentrations, will cause the gland to enlarge. They are:(i) thyroid stimulating hormone (TSH), which stimulates the secretion of the thyroid hormones;(ii) adrenocorticotrophic hormone (ACTH), which acts on the adrenal cortex to promote the release of cortisol;(iii) gonadotrophins LH and FSH, which act on the ovaries and testes. They are however named after their effects in women; FSH stimulates growth of the ovarian follicle containing the ovum or egg and LH stimulates production of oestrogen and progesterone from the ovary. The actions in the male are analogous; FSH stimulates sperm production and LH stimulates testosterone production by the testes.

Prolactin

acts chiefly to cause milk production in the breasts.

Growth hormone

has widespread effects, necessary not only for growth itself but also for metabolism throughout life.

Because the pituitary controls so many endocrine functions in the body it has been called ‘the conductor of the endocrine orchestra’, but more recent discoveries suggested that this term more properly belongs to the hypothalamus, with the pituitary being comparable to the leader of the orchestra. Since the nerves going to the anterior pituitary only supply the blood vessels there was some debate as to how the gland was controlled. It is now known that the hypothalamus produces stimulatory and inhibitory hormones, and that these reach the anterior pituitary via a network of small blood vessels or capillaries. The hormones are produced in nerve cells whose endings abut on the capillaries at the top of the pituitary stalk. This control of the pituitary by the central nervous system allows blood concentrations of the hormones to respond to a variety of external stimuli including stress. It also allows for complex patterning of release. Pituitary hormones in general are released in a pulsatile fashion, with many pulses during the day, and they can also show 24 hour (diurnal) rhythms. The gonadotrophins, linked into the human menstrual cycle, show a 28 day rhythm, while in animals which are seasonal breeders prolactin shows a seasonal rhythm. Blood concentrations of pituitary hormones are controlled not only by the hypothalamic hormones but by feedback, usually negative, exerted by target organ hormones such as cortisol or progesterone.

The posterior pituitary

Two hormones are released from the posterior lobe, oxytocin and vasopressin (syn. antidiuretic hormone). These, like the releasing hormones that reach the anterior lobe, are produced within nerve cells in the hypothalamus. But in this case the axons travel right down the pituitary stalk, and the nerve endings release the hormones directly into the bloodstream (see endocrine). The activity of the posterior pituitary hormones was established around 1900 in the UK by Schafer (a physiologist) and his colleagues working on what proved to be the actions of vasopressin, and Dale, a pharmacologist and Nobel Prize winner working on oxytocin. Vasopressin plays a role in water balance and the maintenance of blood pressure, normal circulating concentrations causing water to be retained by the kidney and higher concentrations causing blood vessels to constrict, thus raising blood pressure. As with the anterior pituitary, control via the hypothalamus means that release of posterior pituitary hormones can be regulated by a variety of nervous inputs; the main stimuli for vasopressin release are an increase in the concentration of the blood plasma and a decrease in circulating blood volume, both of which reflect a fall in total body water. Oxytocin is important for the birth of an infant and for delivery of the milk supply.

Mary L. Forsling

See endocrine.See also growth hormone; hormones; hypothalamus; oxytocin; peptides; water balance.