pituitary gland

n.
A small oval endocrine gland attached to the base of the vertebrate brain and consisting of an anterior and a posterior lobe, the secretions of which control the other endocrine glands and influence growth, metabolism, and maturation. Also called hypophysis, pituitary body.

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The most structurally and functionally complex organ of the endocrine system. Through its hormones, the pituitary, also known as the hypophysis, affects every physiological process of the body. All vertebrates have a pituitary gland with a common basic structure and function. In addition to its endocrine functions, the pituitary may play a role in the immune response.

The hypophysis of all vertebrates has two major segments—the neurohypophysis (a neural component) and the adenohypophysis (an epithelial component)—each with a different embryological origin. The neurohypophysis develops from a downward process of the diencephalon (the base of the brain), whereas the adenohypophysis originates as an outpocketing of the primitive buccal epithelium, known as Rathke's pouch. The adenohypophysis has three distinct subdivisions: the pars tuberalis, the pars distalis, and the pars intermedia. The neurohypophysis comprises the pars nervosa and the infundibulum. The latter consists of the infundibular stalk and the median eminence of the tuber cinereum.

The structural intimacy of neurohypophysis and adenohypophysis that is established early during embryogenesis reflects the direct functional interaction between the central nervous system and endocrine system. The extent of this anatomical intimacy varies considerably among the vertebrate classes, from limited contact to intimate interdigitation. Vascular or neuronal pathways, or both, provide the means of exchanging chemical signals, thus enabling centers in the brain to exert control over the synthesis and release of adenohypophysial hormones.

Neurohormones, which are synthesized in specific regions of the brain, are conveyed to the neurohypophysis by way of axonal tracts, where they may be stored in distended axonal endings. Axons may also contact blood vessels and discharge their neurosecretory products into the systemic circulation or into a portal system leading to the adenohypophysis, or they may directly innervate pituitary gland cells. See also Neurosecretion.

In most animals, the vascular link is the prime route of information transfer between brain and pituitary gland. This link begins in the tuber cinereum, the portion of the third ventricle floor that extends toward the infundibulum. The lower tuber cinereum, which is known as the median eminence, is well endowed with blood vessels that drain down into the pituitary stalk and ultimately empty into the anterior pituitary. The vascular link between the median eminence and the pituitary gland is known as the hypothalamo-hypophysial portal system. The median eminence in humans is vascularized by the paired superior hypophysial arteries. The pituitary gland is believed to have the highest blood flow rate of any organ in the body. However, its blood is received indirectly via the median eminence and the hypothalamo-hypophysial portal system. Most of the blood flow is from the brain to the pituitary gland, with retrograde flow from the adenohypophysis to the hypothalamus, suggesting a two-way communication between nervous and endocrine systems. Although the brain is protected from the chemical substances in the circulatory system by the blood–brain barrier, the median eminence lies outside that protective mechanism and is therefore permeable to intravascular substances. See also Brain.

The hormones of the adenohypophysis may be grouped into three categories based on chemical and functional similarities. The first category consists of growth hormone (also known as somatotropin) and prolactin, both of which are large, single, polypeptide chains; the second category consists of the glycoprotein hormones; this family of hormones contains the gonadotropins and thyrotropin. The gonadotropins in many species, including humans, can be segregated into two distinct hormones, follicle-stimulating hormone and luteinizing hormone. The third group comprises adrenocortiotropic hormone and melanotropin (MSH; melanocyte-stimulating hormone). See also Adenohypophysis hormone.

The regulation of the release of pituitary hormones is determined by precise monitoring of circulating hormone levels in the blood and by genetic and environmental factors that manifest their effect through the releasing and release-inhibiting factors of the hypothalamus. The hypothalamus is located at the base of the brain (the diencephalon) below the thalamus and above the pituitary gland, forming the walls and the lower portion of the third ventricle. It receives major neuronal inputs from the sense organs, hippocampus, thalamus, and lower brainstem structures, including the reticular formation and the spinal cord. Thus, the hypothalamus is designed and anatomically positioned to receive a diversity of messages from external and internal sources that can be transmitted by way of hypothalamic releasing factors to the pituitary gland, where they are translated into endocrine action. See also Nervous system (vertebrate).

The neurohypophysis hormones, oxytocin and vasopressin, are synthesized in different neurons of the paraventricular and supraoptic nuclei of the hypothalamus and travel by axonal flow to the terminals in the neurohypophysis for storage and ultimate release into the vascular system. Oxytocin is important in stimulating milk release through its contractile action on muscle elements in the mammary gland. It also stimulates uterine smooth muscle contraction at parturition. Vasopressin affects water retention by its action on certain kidney tubules. Thus, it also affects blood pressure. See also Lactation; Neurohypophysis hormone.

The better-known neurotransmitters of the central nervous system include the catecholamines (dopamine, epinephrine, and norepinephrine), serotonin, acetylcholine, gamma-amino butyric acid (GABA), histamine, and the opioid peptides (enkephalins, endorphins, dynorphin, neoendorphin, rimorphin, and leumorphin). These substances are distributed widely in the central nervous system and, for most, also in the pituitary gland. If a particular amine or neurotransmitter is present in nerve fibers leading to the median eminence, it probably will influence pituitary gland activity via the portal system. Dopamine, serotonin, gamma-amino butyric acid, and acetylcholine are best known for such activity. These neurotransmitters play an important, but poorly understood, role in regulating pituitary function, either directly or by their action on neuropeptide-producing neurons. Understanding the pharmacology of neurotransmitters holds promise for the treatment of basic disorders of the hypothalamic-pituitary axis. See also Acetylcholine; Endocrine system (vertebrate); Endorphins; Histamine; Hormone; Neurobiology; Neuroimmunology; Pituitary gland disorders; Serotonin.

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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.

n

(hypophysis), an endocrine gland located at the base of the brain in the sella turcica. The pituitary gland is composed of two parts: the pars nervosa, which is an extension of the anterior part of the hypothalamus, and the pars intermedia, which is an epithelial evagination of secretory tissue from the stomodeum of the embryo. By its structural and functional relationships with the nervous system and the endocrine glands, it acts as a mediator of both the nervous system and the endocrine system.

An endocrine gland that coordinates the activities of many other endocrine glands. The pituitary gland is derived from and attached to the base of the brain. The activity of the pituitary gland is largely controlled by inhibiting and releasing factors secreted by the hypothalamus. Hormones produced by the pituitary include adrenocorticotropio (ACTH), follicle stimulating hormone (FSH), growth hormone (GH), and thyroid-stimulating hormone (TSH).

(pi-tooh-uh-ter-ee)

A small gland, attached to the base of the brain and controlled by the hypothalamus, that functions in the endocrine system. The pituitary gland secretes many hormones: some control the actions of other glands, whereas others influence growth, metabolism, and reproduction.

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