pituitary gland

pituitary gland, small oval endocrine gland that lies at the base of the brain. It is sometimes called the master gland of the body because all the other endocrine glands depend on its secretions for stimulation (see endocrine system).

Anatomy and Function

Physiologically, the pituitary is divided into two distinct lobes that arise from different embryological sources. The anterior lobe, or adenohypophysis, grows upward from the pharyngeal tissue at the roof of the mouth. An intermediate lobe also originates in the pharynx, but in humans it is greatly reduced in structure and function. The posterior lobe, or neurohypophysis, grows downward from neural tissue. It is structurally continuous with the hypothalamus of the brain, to which it remains attached by the hypophyseal, or pituitary, stalk. The hypothalamus controls almost all secretions of the pituitary. The posterior lobe is controlled by nerve fibers that originate in hypothalamic neurons and the anterior lobe by substances that are transported from the hypothalamus by tiny blood vessels.

Pituitary Hormones

The tissues in the anterior lobe consist of extensive vascular areas interspersed among glandular cells that secrete at least six different hormones. It was formerly believed that a master molecule was stimulated by various enzymes to produce these hormones, but present evidence indicates that each is individually synthesized, probably by a specific type of glandular cell. Three such types of cells exist in the anterior pituitary gland: acidophils, basophils, and chromophobes. The growth hormone, thought to be synthesized by certain acidophils, stimulates all the tissues in the body to grow by effecting protein formation.

The remaining five important hormones influence body functions by stimulating target organs. Adrenocorticotropic hormone (ACTH) controls the secretion of steroid hormones by the adrenal cortex, which affects glucose, protein, and fat metabolism; thyrotropin controls the rate of thyroxine synthesis by the thyroid gland, which is the principal regulator of body metabolic rate; prolactin, which regulates the formation of milk after the birth of an infant; and three separate gonadotropic hormones (follicle-stimulating hormone, luteinizing hormone, and luteotropic hormone) control the growth and reproductive activity of the gonads.

The release of each of the hormones from the anterior lobe is controlled by a specific substance secreted by nerve cells in the hypothalamus. These substances, called releasing factors, are transmitted by nerve fibers to tiny capillaries in the hypophyseal stalk. They move through blood vessels to the anterior lobe, where each releasing factor is responsible for the release of a specific pituitary hormone.

The two hormones that are produced by the posterior lobe are synthesized by nerve cells in the hypothalamus. They are transported by nerve fibers to nerve endings in the posterior lobe, where they are released. The hormones are antidiuretic hormone (ADH or vasopressin), which alters the permeability of the kidney tubules, permitting more water to be retained by the body; and oxytocin, which aids in the release of milk from mammary glands and causes uterine contractions. The only hormone that is synthesized by the intermediate lobe is the melanocyte-stimulating hormone, which appears to control skin pigmentation.

Disorders of Pituitary Hormone Secretion

Oversecretion of the pituitary hormone human growth hormone can cause gigantism if it occurs before growth of the long bones is complete, or acromegaly if it begins during adulthood. Undersecretion of human growth hormone can lead to dwarfism if experienced during childhood, and decreased endocrine function accompanied by lethargy and loss of sexual capacity in the adult.

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pituitary gland

 or hypophysis

Endocrine gland lying on the underside of the brain that plays a major role in regulating the endocrine system. The anterior pituitary lobe secretes six hormones that play specific roles in stimulating production of cortisol and androgens by the adrenal cortex (corticotropin), growth of eggs and sperm (follicle-stimulating hormone), production of progesterone and testosterone (luteinizing hormone), linear growth in children and bone maintenance in adults (growth hormone), milk production (prolactin), and production of thyroid hormone (thyrotropin). The posterior lobe stores and releases two hormones, oxytocin and vasopressin, from nerve cells in specialized regions of the hypothalamus that control pituitary function. These hormones stimulate uterine contraction and milk secretion (oxytocin) and blood pressure and fluid balance (vasopressin).

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pituitary gland, body

the master endocrine gland, attached by a stalk to the base of the brain. Its two lobes (see adenohypophysis and neurohypophysis) secrete hormones affecting skeletal growth, development of the sex glands, and the functioning of the other endocrine glands Also called hypophysis hypophysis cerebri

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Pituitary gland

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 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 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 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 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 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 Acetylcholine, Endocrine system (vertebrate), Endorphins, Histamine, Hormone, Neurobiology, Neuroimmunology, Serotonin