Pituitary gland

Definition

The pituitary gland is located at the base of the brain and is part of the endocrine system . It is sometimes called the hypophysis, from two Greek words that mean "to grow beneath." The pituitary is responsible for the hormonal regulation of several body processes, including water retention, breast milk synthesis and release, human growth, and thyroid gland secretions.

Description

The pituitary is one of the most extensively researched glands in the endocrine system. In humans, it is located at the base of the brain just beneath the hypothalamus. There are three separate lobes (or sections) of the pituitary: the anterior lobe, the posterior lobe and the intermediate lobe. Therefore, it is sometimes considered as three different glands. In addition, there is a small stem called the pituitary stalk that connects the pituitary to the hypothalamus.

The pituitary gland is formed during early fetal development . An understanding of its formation explains its position in the endocrine system as well as its neurological importance. Early in the development of the fetus, a small sac of cells forms at the top of the oral cavity and moves upward. These cells are known as Rathke's pouch. At the same time, a small fold of neural tissue extends downward from the hypothalamus. During fetal development, the two structures continue to move toward each other; they meet and fuse to form the anterior (originally Rathke's pouch) and posterior (from the hypothalamus) lobes of the pituitary.

The hypothalamus, which is located just above the pituitary gland, is a region in the forebrain that is responsible for regulating all lobes of the pituitary. The pituitary releases, but does not necessarily synthesize, nine different hormones. Neurohormones are synthesized by the hypothalamus and transported to the posterior pituitary. The release of hormones from each lobe of the pituitary is regulated differently.

Anterior pituitary

The anterior pituitary is sometimes called the adenohypophysis. It constitutes about 80% of the pituitary by weight. The cells of the anterior pituitary act like true endocrine cells. Instead of containing neurons , the anterior pituitary receives chemical signals through the blood and releases hormones in response. It has a direct connection with the hypothalamus through blood vessels . Various cells in the anterior pituitary release the following hormones:

• Gonadotrophs release luteinizing hormone (LH) and follicle-stimulating hormone (FSH).
• Lactotrophs release prolactin (PRL).
• Corticotrophs release adrenocorticotropic hormone (ACTH).
• Somatotrophs release growth hormone (GH).
• Thyrotrophs release thyroid-stimulating hormone (TSH).

Posterior pituitary

The posterior pituitary is sometimes referred to as the neurohypophysis because it acts like an extension of the nervous system. As opposed to the anterior pituitary, which is connected to the hypothalamus via the circulatory system, the posterior pituitary receives nerve impulses from the same nerve cells that innervate the hypothalamus. The posterior pituitary releases oxytocin and antidiuretic hormone (ADH, or vasopressin).

Intermediate pituitary

The intermediate lobe is not a complete "lobe" in humans. Instead it is a simple structure comprised of just a few cells. The intermediate pituitary is an important structure in many lower vertebrates, but it has very little significance in humans. In lower vertebrates, the intermediate

KEY TERMS

Acromegaly —Abnormal enlargement of such parts of the body as the hands, face, head, and feet.

Adenohypophysis —Another name for the anterior lobe of the pituitary.

Antidiuretic hormone (ADH) —A hormone released by the posterior lobe of the pituitary gland that increases the absorption of water by the kidneys. It is also known as vasopressin.

Diabetes insipidus —A disorder characterized by increased urine production resulting from inadequate levels of ADH.

Gonadotropins —Hormones that affect the development or activity of the ovaries or testes.

Hypophysis —Another name for the pituitary gland.

Hypopituitarism —A condition produced by deficient activity of the anterior lobe of the pituitary gland. It is characterized by obesity, incomplete sexual maturation, and in extreme cases, dwarfism.

Hypothalamus —A region in the forebrain that regulates the functions of the autonomic nervous system by vascular communication. The hypothalamus governs the functions of both lobes of the pituitary gland.

Neurohypophysis —Another name for the posterior lobe of the pituitary.

Oxytocin —A hormone produced by the posterior pituitary that stimulates the movement of breast milk from the sacs in which the milk is produced to the larger ducts from which the milk is ejected.

Peptide —A compound containing two or more amino acids, in which the carboxyl group of one acid is linked to the amino group of the other.

Vasopressin —Another name for ADH.

pituitary releases melanocyto-stimulating hormone. This hormone stimulates the growth of melanocytes, which are cells that produce a dark pigment called melanin.

Function

The pituitary gland is an organ that is part of the endocrine system, along with many other glands and organs. It is regulated by the hypothalamus, and it in turn regulates the secretion of many different hormones that are essential to human health.

Role in human health

Hormones released from the anterior and posterior pituitary have far-reaching effects on many different organ systems and physiological processes.

Hormones of the anterior pituitary

Luteinizing hormone and follicle stimulating hormone are called gonadotropins. As the name suggests, the target tissues of these hormones are the gonads (ovaries and testes). They have two main functions. The first is to promote the development and maturation of sperm and eggs. Second, they stimulate the production and release of such sex steroid hormones as estradiol and testosterone in women and men respectively.

Prolactin is responsible for stimulating cells in the female breast to produce milk. Therefore, lactotrophs located in the anterior pituitary of women that are breast-feeding are large and numerous, indicating an increased amount of prolactin production. These lactotrophs comprise about 30% of the cells in the anterior pituitary. The pituitary in women doubles in size during pregnancy because of the increase in size and number of lactotrophs.

The target tissue of adrenocorticotropic hormone is the adrenal cortex (part of the adrenal gland that is located above the kidney). ACTH stimulates the production of cortisol and also causes the cells of the adrenal gland to grow. Cortisol has many effects on metabolism in various tissues.

Growth hormones have many different target tissues and promote the growth of each of them. For this reason human growth hormone (GH) is considered an anabolic hormone, indicating that it is responsible for building tissue proteins . For example, GH directly increases protein synthesis in muscles and the liver ; and it decreases the size of adipose tissue. It also has an indirect effect by stimulating other hormones. Growth hormones indirectly affect the bones by increasing protein synthesis, collagen synthesis and cell proliferation. In many other tissues, the indirect effects of growth hormone are responsible for protein, RNA and DNA synthesis. The overall effect of growth hormone is to promote skeletal growth and a lean body mass.

As its name implies, thyroid-stimulating hormone (TSH) promotes cell growth in the thyroid gland. TSH also triggers the secretion of thyroid hormones that affect many metabolic processes in the body.

Hormones of the posterior pituitary

Both oxytocin and antidiuretic hormone (ADH) are peptide hormones that are synthesized in the cell bodies of the nerves originating in the hypothalamus and then delivered through the axons to the posterior pituitary. Thus, they are good examples of neuroendocrine hormones.

The primary target organ of ADH is the kidney. ADH is responsible for increasing water retention by the kidney, resulting in an increase in extracellular fluid and a decrease in urine volume. Receptors in the hypothalamus called osmoreceptors can sense the concentration of water in the extracellular fluid through changes in extracellular fluid osmolarity. The osmoreceptors in turn determine the release of ADH by the posterior pituitary. The consumption of alcohol decreases the amount of ADH released. As a result, more fluid is lost through urination, resulting in excessive water loss and thirst.

The primary site of action of oxytocin is female breast tissue. Oxytocin stimulates the contraction of smooth muscle cells in the breast, transferring milk from the place of synthesis to the larger ducts of the breast. Oxytocin is secreted by the stimulation of touch sensors when an infant is suckling. Other psychological factors, such as the sound of a baby crying, can stimulate the release and action of oxytocin. The role of oxytocin in the onset of labor contractions is not fully clear. There is no known stimulus for the secretion of oxytocin in the human male.

Common diseases and disorders

Hypopituitarism

Disorders of the pituitary gland can have severe effects on normal growth and sexual maturation. A general condition known as hypopituitarism , also known as pituitary dwarfism, is characterized by a decrease in one or more of the hormones produced by the anterior pituitary. Sexual immaturity and metabolic dysfunction leading to obesity are symptoms of this syndrome. When hypopituitarism occurs in childhood, growth is slowed. Tumors are often the cause of hypopituitarism; however, sometimes there is no identifiable cause. If there is a decrease in the levels of hormones released from the hypothalamus, then hypopituitarism results. The symptoms vary according to the number and amount of hormones that are deficient. The most effective treatment is the administration of replacement hormones.

Hyperpituitarism

The overproduction of growth hormone during childhood produces a condition known as gigantism or acromegaly. Excessive secretion of anterior pituitary hormones is known as hyperpituitarism. Growth hormone influences the overgrowth of the skeleton and all other tissues. A person may grow to 8 ft (2.4 m) or more in height. It is still unclear, but researchers think that over-production of growth hormone may be caused by an adenoma (tumor) on the anterior pituitary. Sometimes, this condition occurs in more than one member of the family, suggesting that there is a genetic component. Gigantism is treated by removing the tumor and administering medications (bromocriptine and octreotide) that inhibit the production of growth hormone.

Diabetes insipidus

A disorder related to both the hypothalamus and the posterior lobe of the pituitary is diabetes insipidus, not to be confused with diabetes mellitus . Diabetes insipidus, or DI, is caused by a deficiency of antidiuretic hormone (ADH). As a result, water is rapidly released from the body through large volumes of urine (3–30 quarts per day). DI may result from an inherited trait; from damage to the hypothalamus, which synthesizes ADH; or from damage to the posterior pituitary, which stores the ADH. Diabetes insipidus occurs more frequently in men than in women. In mild cases, no treatment is necessary other than water replacement. In extreme cases, the patient can be treated by hormone replacement therapy.

Resources

BOOKS

Cahill, Matthew, ed. Professional Guide to Diseases. 6th ed. Springhouse, PA: Springhouse Corporation, 1998.

Greenspan, Francis S., and David G. Gardner. Basic and Clinical Endocrinology. 6th ed. New York: Lange Medical Books/McGraw-Hill, 2001.

"Hypothalamic-Pituitary Relationships." Chapter 6 in The Merck Manual of Diagnosis and Therapy, edited by Mark H. Beers, MD, and Robert Berkow, MD. Whitehouse Station, NJ: Merck Research Laboratories, 1999.

Martin, John H., PhD. Neuroanatomy: Text and Atlas. 2nd ed. Norwalk, CT: Appleton & Lange, 1996.

"Pituitary Disorders." Chapter 7 in The Merck Manual of Diagnosis and Therapy, edited by Mark H. Beers, MD, and Robert Berkow, MD. Whitehouse Station, NJ: Merck Research Laboratories, 1999.

Vander, Arthur, et al, eds. Human Physiology: The Mechanisms of Body Function, 7th ed. Boston, MA: WBC/McGraw-Hill, 1998.

Sally C. McFarlane-Parrott

Pituitary Gland

Definition

The pituitary gland is located at the base of the brain and is part of the endocrine system. It is sometimes called the hypophysis, from two Greek words that mean "to grow beneath." The pituitary is responsible for the hormonal regulation of several body processes, including water retention, breast milk synthesis and release, human growth, and thyroid gland secretions.

Description

The pituitary is one of the most extensively researched glands in the endocrine system. In humans, it is located at the base of the brain just beneath the hypothalamus. There are three separate lobes (or sections) of the pituitary: the anterior lobe, the posterior lobe and the intermediate lobe. Therefore, it is sometimes considered as three different glands. In addition, there is a small stem called the pituitary stalk that connects the pituitary to the hypothalamus.

The pituitary gland is formed during early fetal development. An understanding of its formation explains its position in the endocrine system as well as its neurological importance. Early in the development of the fetus, a small sac of cells forms at the top of the oral cavity and moves upward. These cells are known as Rathke's pouch. At the same time, a small fold of neural tissue extends downward from the hypothalamus. During fetal development, the two structures continue to move toward each other; they meet and fuse to form the anterior (originally Rathke's pouch) and posterior (from the hypothalamus) lobes of the pituitary.

The hypothalamus, which is located just above the pituitary gland, is a region in the forebrain that is responsible for regulating all lobes of the pituitary. The pituitary releases, but does not necessarily synthesize, nine different hormones. Neurohormones are synthesized by the hypothalamus and transported to the posterior pituitary. The release of hormones from each lobe of the pituitary is regulated differently.

Anterior pituitary

The anterior pituitary is sometimes called the adenohypophysis. It constitutes about 80% of the pituitary by weight. The cells of the anterior pituitary act like true endocrine cells. Instead of containing neurons, the anterior pituitary receives chemical signals through the blood and releases hormones in response. It has a direct connection with the hypothalamus through blood vessels. Various cells in the anterior pituitary release the following hormones:

• Gonadotrophs release luteinizing hormone (LH) and follicle-stimulating hormone (FSH).
• Lactotrophs release prolactin (PRL).
• Corticotrophs release adrenocorticotropic hormone (ACTH).
• Somatotrophs release growth hormone (GH).
• Thyrotrophs release thyroid-stimulating hormone (TSH).

Posterior pituitary

The posterior pituitary is sometimes referred to as the neurohypophysis because it acts like an extension of the nervous system. As opposed to the anterior pituitary, which is connected to the hypothalamus via the circulatory system, the posterior pituitary receives nerve impulses from the same nerve cells that innervate the hypothalamus. The posterior pituitary releases oxytocin and antidiuretic hormone (ADH, or vasopressin).

Intermediate pituitary

The intermediate lobe is not a complete "lobe" in humans. Instead it is a simple structure comprised of just a few cells. The intermediate pituitary is an important structure in many lower vertebrates, but it has very little significance in humans. In lower vertebrates, the intermediate pituitary releases melanocyto-stimulating hormone. This hormone stimulates the growth of melanocytes, which are cells that produce a dark pigment called melanin.

Function

The pituitary gland is an organ that is part of the endocrine system, along with many other glands and organs. It is regulated by the hypothalamus, and it in turn regulates the secretion of many different hormones that are essential to human health.

Role in human health

Hormones released from the anterior and posterior pituitary have far-reaching effects on many different organ systems and physiological processes.

Hormones of the anterior pituitary

Luteinizing hormone and follicle stimulating hormone are called gonadotropins. As the name suggests, the target tissues of these hormones are the gonads (ovaries and testes). They have two main functions. The first is to promote the development and maturation of sperm and eggs. Second, they stimulate the production and release of such sex steroid hormones as estradiol and testosterone in women and men respectively.

Prolactin is responsible for stimulating cells in the female breast to produce milk. Therefore, lactotrophs located in the anterior pituitary of women that are breastfeeding are large and numerous, indicating an increased amount of prolactin production. These lactotrophs comprise about 30% of the cells in the anterior pituitary. The pituitary in women doubles in size during pregnancy because of the increase in size and number of lactotrophs.

The target tissue of adrenocorticotropic hormone is the adrenal cortex (part of the adrenal gland that is located above the kidney). ACTH stimulates the production of cortisol and also causes the cells of the adrenal gland to grow. Cortisol has many effects on metabolism in various tissues.

Growth hormones have many different target tissues and promote the growth of each of them. For this reason human growth hormone (GH) is considered an anabolic hormone, indicating that it is responsible for building tissue proteins. For example, GH directly increases protein synthesis in muscles and the liver; and it decreases the size of adipose tissue. It also has an indirect effect by stimulating other hormones. Growth hormones indirectly affect the bones by increasing protein synthesis, collagen synthesis and cell proliferation. In many other tissues, the indirect effects of growth hormone are responsible for protein, RNA and DNA synthesis. The overall effect of growth hormone is to promote skeletal growth and a lean body mass.

As its name implies, thyroid-stimulating hormone (TSH) promotes cell growth in the thyroid gland. TSH also triggers the secretion of thyroid hormones that affect many metabolic processes in the body.

Hormones of the posterior pituitary

Both oxytocin and antidiuretic hormone (ADH) are peptide hormones that are synthesized in the cell bodies of the nerves originating in the hypothalamus and then delivered through the axons to the posterior pituitary. Thus, they are good examples of neuroendocrine hormones.

The primary target organ of ADH is the kidney. ADH is responsible for increasing water retention by the kidney, resulting in an increase in extracellular fluid and a decrease in urine volume. Receptors in the hypothalamus called osmoreceptors can sense the concentration of water in the extracellular fluid through changes in extracellular fluid osmolarity. The osmoreceptors in turn determine the release of ADH by the posterior pituitary. The consumption of alcohol decreases the amount of ADH released. As a result, more fluid is lost through urination, resulting in excessive water loss and thirst.

The primary site of action of oxytocin is female breast tissue. Oxytocin stimulates the contraction of smooth muscle cells in the breast, transferring milk from the place of synthesis to the larger ducts of the breast. Oxytocin is secreted by the stimulation of touch sensors when an infant is suckling. Other psychological factors, such as the sound of a baby crying, can stimulate the release and action of oxytocin. The role of oxytocin in the onset of labor contractions is not fully clear. There is no known stimulus for the secretion of oxytocin in the human male.

Common diseases and disorders

Hypopituitarism

Disorders of the pituitary gland can have severe effects on normal growth and sexual maturation. A general condition known as hypopituitarism, also known as pituitary dwarfism, is characterized by a decrease in one or more of the hormones produced by the anterior pituitary. Sexual immaturity and metabolic dysfunction leading to obesity are symptoms of this syndrome. When hypopituitarism occurs in childhood, growth is slowed. Tumors are often the cause of hypopituitarism; however, sometimes there is no identifiable cause. If there is a decrease in the levels of hormones released from the hypothalamus, then hypopituitarism results. The symptoms vary according to the number and amount of hormones that are deficient. The most effective treatment is the administration of replacement hormones.

Hyperpituitarism

The overproduction of growth hormone during childhood produces a condition known as gigantism or acromegaly. Excessive secretion of anterior pituitary hormones is known as hyperpituitarism. Growth hormone influences the overgrowth of the skeleton and all other tissues. A person may grow to 8 ft (2.4 m) or more in height. It is still unclear, but researchers think that overproduction of growth hormone may be caused by an adenoma (tumor) on the anterior pituitary. Sometimes, this condition occurs in more than one member of the family, suggesting that there is a genetic component. Gigantism is treated by removing the tumor and administering medications (bromocriptine and octreotide) that inhibit the production of growth hormone.

KEY TERMS

Acromegaly— Abnormal enlargement of such parts of the body as the hands, face, head, and feet.

Adenohypophysis— Another name for the anterior lobe of the pituitary.

Antidiuretic hormone (ADH)— A hormone released by the posterior lobe of the pituitary gland that increases the absorption of water by the kidneys. It is also known as vasopressin.

Diabetes insipidus— A disorder characterized by increased urine production resulting from inadequate levels of ADH.

Gonadotropins— Hormones that affect the development or activity of the ovaries or testes.

Hypophysis— Another name for the pituitary gland.

Hypopituitarism— A condition produced by deficient activity of the anterior lobe of the pituitary gland. It is characterized by obesity, incomplete sexual maturation, and in extreme cases, dwarfism.

Hypothalamus— A region in the forebrain that regulates the functions of the autonomic nervous system by vascular communication. The hypothalamus governs the functions of both lobes of the pituitary gland.

Neurohypophysis— Another name for the posterior lobe of the pituitary.

Oxytocin— A hormone produced by the posterior pituitary that stimulates the movement of breast milk from the sacs in which the milk is produced to the larger ducts from which the milk is ejected.

Peptide— A compound containing two or more amino acids, in which the carboxyl group of one acid is linked to the amino group of the other.

Vasopressin— Another name for ADH.

Diabetes insipidus

A disorder related to both the hypothalamus and the posterior lobe of the pituitary is diabetes insipidus, not to be confused with diabetes mellitus. Diabetes insipidus, or DI, is caused by a deficiency of antidiuretic hormone (ADH). As a result, water is rapidly released from the body through large volumes of urine (3-30 quarts per day). DI may result from an inherited trait; from damage to the hypothalamus, which synthesizes ADH; or from damage to the posterior pituitary, which stores the ADH. Diabetes insipidus occurs more frequently in men than in women. In mild cases, no treatment is necessary other than water replacement. In extreme cases, the patient can be treated by hormone replacement therapy.

Resources

BOOKS

Cahill, Matthew, ed. Professional Guide to Diseases, 6th ed. Springhouse, PA: Springhouse Corporation, 1998.

Greenspan, Francis S., and David G. Gardner. Basic and Clinical Endocrinology, 6th ed. New York: Lange Medical Books/McGraw-Hill, 2001.

"Hypothalamic-Pituitary Relationships." Chapter 6 in The Merck Manual of Diagnosis and Therapy, edited by Mark H. Beers, MD, and Robert Berkow, MD. Whitehouse Station, NJ: Merck Research Laboratories, 1999.

Martin, John H., PhD. Neuroanatomy: Text and Atlas, 2nd ed. Norwalk, CT: Appleton & Lange, 1996.

"Pituitary Disorders." in The Merck Manual of Diagnosis and Therapy, edited by Mark H. Beers, MD, and Robert Berkow, MD. Whitehouse Station, NJ: Merck Research Laboratories, 2004.

Vander, Arthur, et al, eds. Human Physiology: The Mechanisms of Body Function, 7th ed. Boston, MA: WBC/McGraw-Hill, 1998.

Pituitary Gland

The pituitary gland is one of the principal glands of the endocrine system. It releases at least nine hormones affecting a wide variety of body functions, including growth, reproduction, and levels of electrolytes and water in the body fluids. The pituitary sits near the center of the head, behind the nose and beneath the brain, just below the hypothalamus. The hypothalamus is a brain structure from which the pituitary receives chemical signals that control its action. Nerve endings from the hypothalamus stimulate the posterior portion of the pituitary to secrete oxytocin and antidiuretic hormone (ADH). Capillaries from the hypothalamus carry releasing factors and inhibiting factors to the anterior portion of the pituitary, stimulating or inhibiting release of eight other hormones (see Table 1). All the hormones of the pituitary gland are peptides, small chains of amino acids .

Both the hypothalamus and the pituitary are involved in complex feedback loops with other glands in the body, sending and receiving hormonal signals to maintain homeostasis. Because of its central role in so many systems, pituitary abnormalities can lead to a variety of disorders. Disorders may lead to either hyposecretion or hypersecretion . Deficient growth hormone, for instance, leads to dwarfism, while excess causes gigantism.

see also Endocrine System; Growth; Homeostasis; Hormones; Hypothalamus

Richard Robinson

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

pituitary gland (pituitary body) A gland at the base of the brain that secretes a range of hormones, mostly tropins. It is the dominant gland of, and regulates, the endocrine system and also interacts with the hypothalamus in the pituitary axis. The gland comprises an anterior lobe and a posterior lobe. The anterior lobe secretes gonadotropins, thyrotropic hormone, and adrenocorticotrophic hormone; the posterior lobe secretes oxytocin and vasopressin. See also ADENOHYPOPHYSIS; NEUROHYPOPHYSIS.