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Anatomy and Function
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
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
(glandula pituitaria; hypophysis cerebri), an endocrine gland that plays an important part in hormonal regulation in all vertebrate animals and man.
The pituitary gland is located at the base of the brain in the sella turcica of the sphenoid bone and connected to the base by the infundibulum, an outgrowth of the floor of the third cerebral ventricle. The size, shape, and weight of the pituitary varies by species and depends upon the age and physiological state of the organism; it weighs 0.5-0.6 g in man. The gland has three lobes: anterior (glandular), intermediate, and posterior (neural). The anterior and intermediate lobes originate in the embryo as an evagination of the epithelium of the roof of the primitive oral cavity. The posterior lobe is formed from the floor of the infundibulum of the diencephalon. The embryonic rudiment of the anterior and intermediate lobes subsequently separates from the epithelium of the primitive oral cavity, grows toward the brain, and unites with the rudiment of the posterior lobe. Only in some cartilaginous fishes does the attachment of the anterior lobe to the epithelium of the primitive oral cavity survive in the adult organism. In certain mammals (for example, in the cats) the posterior lobe of the pituitary has a cavity that communicates with the cavity of the third ventricle; in other mammals (for example, in dogs) the cavity survives only in the stalk that connects the pituitary to the diencephalon. In still other mammals (for example, in the rabbit and all primates) the posterior lobe and stalk of the pituitary are solid formations and lack any cavity. In adult organisms the pituitary is closely associated anatomically with the brain. It is supplied with a large number of nerve fibers that enter it from the hypothalamic region through the stalk and along the walls of the hypophyseal arteries from the carotid neural plexus.
The anterior lobe of the pituitary gland in the adult consists of adenose epithelium containing three kinds of cells, which differ in their ability to be stained with acid or basic dyes: chromophobic, or chief, cells; oxyphilic, or eosinophilic, cells; and basophilic cells. The chromophobic cells are the reserve material from which the oxyphilic and basophilic cells develop. The proportions of oxyphilic and basophilic cells in the anterior lobe vary with the sex, age, and physiological state of the organism. For example, after removal of the thyroid gland (thyroidectomy), the number of oxyphilic cells decreases sharply, to the point of complete disappearance, while basophilic cells degenerate and are transformed into so-called thyroidectomy cells. After castration basophilic cells hypertrophy and are transformed into so-called castration cells. The changes that take place in the composition of the cells of the anterior pituitary after thyroidectomy or castration can be prevented or corrected by the administration of thyroxine or sex hormones. The intermediate lobe of the pituitary consists of epithelial tissue. The anterior lobe is formed by neuroglia, which contains large pyramidal or spindle cells (so-called pituicytes).
The physiological role of the anterior pituitary is most complex and varied. Growth, reproduction, and basal, carbohydrate, mineral, fat, and protein metabolism depend upon its normal functioning. Seven hormones have been isolated from anterior pituitary extract: growth (somatotropic) hormone, thyrotropic hormone, follicle-stimulating hormone, luteinizing hormone, luteotropic hormone, prolactin (lactogenic hormone), and adrenocorticotropic hormone (ACTH). All these hormones are protein in nature and can be obtained in pure form. Some of them, such as growth hormone and lactogenic hormone, have been isolated in crystalline form, and others, such as ACTH, have been synthesized. Thyrotropic and gonadotropic hormones are produced by the basophilic cells, which are divided accordingly into thyrotrophs and gonadotrophs. The oxyphilic cells manufacture growth hormone and prolactin. The cells that produce ACTH have not been determined, although it is probable that the basophilic cells are involved.
Surgical removal of the pituitary gland (pituitectomy, or hypophysectomy) in young animals causes cessation of growth. Injecting the animals with a pituitary extract that contains growth hormone restores their normal growth. Injection of young growing animals with growth hormone sharply stimulates growth and results in gigantism (giant salamanders, rats, dogs, and other animals have been produced experimentally). In man, excessive secretion of growth hormone causes a disease with signs of gigantism or acromegaly. Insufficient secretion of growth hormone causes dwarfism.
Atrophy of the genital system following removal of the pituitary may be prevented by administering gonadotropic hormones. Injection of young animals with these hormones causes precocious sexual maturity. Injection of frogs with a pituitary extract containing gonadotropic hormones stimulates spawning and spermatogenesis in the fall and winter; normal tadpoles develop from the fertilized eggs. Follicle-stimulating hormone regulates both growth of the follicles in the ovaries of the female and spermatogenesis in the male. Luteinizing hormone causes premature growth of the follicles, ovulation, and formation of the corpus luteum in females, and it stimulates secretion of the male sex hormone by the interstitial (Leydig) cells of the testes in males. Luteotropic hormone maintains the function of the corpus luteum; the hormone causes lactation in some animals, such as rats and sheep. Prolactin (lactogenic hormone) helps to regulate the secretion of milk; removal of the anterior pituitary in lactating females halts milk secretion but introduction of prolactin causes secretion to resume. Removal of the anterior pituitary causes the thyroid to atrophy; as a result, basal metabolism is lowered. Injection of a pituitary extract containing thyrotropic hormone enlarges the thyroid gland and reinforces its proper functioning. ACTH stimulates the adrenal cortex and its secretion of corticosteroid hormones. It also reverses atrophy of the gland following removal of the pituitary. Growth hormone and ACTH are among the hormones of the anterior pituitary that influence metabolism.
The intermediate lobe of the pituitary manufactures the hormone intermedin, or melanin-stimulating hormone, which influences skin color in fish and amphibians. The physiological role of this hormone in birds and mammals is still unclear.
The posterior lobe of the pituitary takes part in the regulation of blood pressure and urine output, through the hormone vasopressin, and activity of the uterine muscles, through the hormone oxytocin. Vasopressin and oxytocin are formed in the paraventricular and supraoptic nuclei of the hypothalamus, from which they enter the posterior lobe of the pituitary. Both hormones have been synthesized.
Pituitary functions depend upon environmental conditions. Experiments performed on birds and mammals have shown that light regulates the gonadotropic, thyrotropic, and adrenocorticotropic functions of the pituitary. The effect of light on the pituitary is mediated by the central nervous system. It has also been proved that the endocrine functions of the pituitary are controlled by the hypothalamus, which produces special neurohumoral substances of a peptide nature (the so-called releasing factors) that stimulate the humoral secretion of pituitary hormones.
Impairment of normal pituitary activity may be manifested by an increase (hyperpituitarism) or reduction (hypopituitarism) of the pituitary’s individual functions and less commonly by the complete loss of those functions. Increased internal secretion of the pituitary results in growth and development disorders, such as gigantism in children and acromegaly in adults. Reduction or loss of pituitary functions in children leads to stunted growth (dwarfism), mental retardation, infantilism, and atrophy of the thyroid gland and adrenal cortex, with profound changes in carbohydrate and fat metabolism and weakening of oxidative processes. In adults it results in obesity, cessation of the sexual cycle, and atrophy of the thyroid, gonads, and adrenal cortex. Primary disturbances of hypothalamic activity are decisive factors in the etiology of a number of so-called pituitary diseases, including Itsenko-Cushing disease, diabetes insipidus, and sexual precocity.
REFERENCESKirshenblat, Ia. D. Obshchaia endokrinologiia. Moscow, 1965.
Gipotalamicheskaia reguliatsiia perednei chasti gipofiza. Budapest, 1965. (Translated from English.)
Leites, S. M., and N. N. Lapteva. Ocherkipo patofiziologii obmena veshchestv i endokrinnoi sistemy. Moscow, 1967.
Eskin, I. A. Osnovy fiziologii endokrinnykh zhelez. Moscow, 1968.
Tonkikh, A. V. Gipotalamo-gipofizarnaia oblast’ i reguliatsiia fiziologicheskikh funktsii organizma. Moscow-Leningrad, 1965.
Schreiber, V. The Hypothalamo-Hypophysial System. Prague, 1963.
I. A. ESKIN and L. M. GOL’BER