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I   INTRODUCTION  

Pituitary Gland, master endocrine gland in vertebrate animals. The hormones secreted by the pituitary stimulate and control the functioning of almost all the other endocrine glands in the body. Pituitary hormones also promote growth and control the water balance of the body. 

The pituitary is a small bean-shaped, reddish-gray organ located in the saddle-shaped depression (sella turcica) in the floor of the skull (the sphenoid bone) and attached to the base of the brain by a stalk; it is located near the hypothalamus. The pituitary has two

lobes—the anterior lobe, or adenohypophysis, and the posterior lobe, or neurohypophysis—which differ in structure and function. The anterior lobe is derived embryologically from the roof of the pharynx and is composed of groups of epithelial cells separated by blood channels; the posterior lobe is derived from the base of the brain and is composed of nervous connective tissue and nervelike secreting cells. The area between the anterior and posterior lobes of the pituitary is called the intermediate lobe; it has the same embryological origin as the anterior lobe.

II THE ANTERIOR LOBE 

 
Concentrated chemical substances, or hormones, which control 10 to 12 functions in the body, have been obtained as extracts from the anterior pituitary glands of cattle, sheep, and swine. Eight hormones have been isolated, purified, and identified; all of them are peptides, that is, they are composed of amino acids. Growth hormone (GH), or the somatotropic hormone (STH), is essential for normal skeletal growth and is neutralized during adolescence by the gonadal sex hormones. Thyroid-stimulating hormone (TSH) controls the normal functioning of the thyroid gland; and the adrenocorticotropic hormone (ACTH) controls the activity of the cortex of the adrenal glands and takes part in the stress reaction . Prolactin, also called lactogenic, luteotropic, or mammotropic hormone, initiates milk secretion in the mammary gland after the mammary tissues have been prepared during pregnancy by the secretion of other pituitary and sex hormones. The two gonadotropic hormones are follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Follicle-stimulating hormone stimulates the formation of the Graafian follicle in the female ovary and the development of spermatozoa in the male. The luteinizing hormone stimulates the formation of ovarian hormones after ovulation and initiates lactation in the female; in the male, it stimulates the tissues of the testes to elaborate testosterone. In 1975 scientists identified the pituitary peptide endorphin, which acts in experimental animals as a natural pain reliever in times of stress. Endorphin and ACTH are made as parts of a single large protein, which subsequently splits. This may be the body's mechanism for coordinating the physiological activities of two stress-induced hormones. The same large prohormone that contains ACTH and endorphin also contains short peptides called melanocyte-stimulating hormones. These substances are analogous to the hormone that regulates pigmentation in fish and amphibians, but in humans they have no known function.

Research has shown that the hormonal activity of the anterior lobe is controlled by chemical messengers sent from the hypothalamus through tiny blood vessels to the anterior lobe. In the 1950s, the British neurologist Geoffrey Harris discovered that cutting the blood supply from the hypothalamus to the pituitary impaired the function of the pituitary. In 1964, chemical agents called releasing factors were found in the hypothalamus; these substances, it was learned, affect the secretion of growth hormone, a thyroid-stimulating hormone called thyrotropin, and the gonadotropic hormones involving the testes and ovaries. In 1969 the American endocrinologist Roger Guillemin and colleagues isolated and characterized thyrotropin-releasing factor, which stimulates the secretion of thyroid-stimulating hormone from the pituitary. In the next few years his group and that of the American physiologist Andrew Victor Schally isolated the luteinizing hormone-releasing factor, which stimulates secretion of both LH and FSH, and somatostatin, which inhibits release of growth hormone. For this work, which proved that the brain and the endocrine system are linked, they shared the Nobel Prize in physiology or medicine in 1977. Human somatostatin was one of the first substances to be grown in bacteria by recombinant DNA.

The presence of the releasing factors in the hypothalamus helped to explain the action of the female sex hormones, estrogen and progesterone, and their synthetic versions contained in oral contraceptives, or birth-control pills. During a woman's normal monthly cycle, several hormonal changes are needed for the ovary to produce an egg cell for possible fertilization. When the estrogen level in the body declines, the follicle-releasing factor (FRF) flows to the pituitary and stimulates the secretion of the follicle-stimulating hormone. Through a similar feedback principle, the declining level of progesterone causes a release of luteal-releasing factor (LRF), which stimulates secretion of the luteinizing hormone. The ripening follicle in the ovary then produces estrogen, and the high level of that hormone influences the hypothalamus to shut down temporarily the production of FSH. Increased progesterone feedback to the hypothalamus shuts down LH production by the pituitary. The daily doses of synthetic estrogen and progesterone in oral contraceptives, or injections of the actual hormones, inhibit the normal reproductive activity of the ovaries by mimicking the effect of these hormones on the hypothalamus.

III THE INTERMEDIATE LOBE

 
In lower vertebrates this part of the pituitary secretes melanocyte-stimulating hormone, which brings about skin-color changes. In humans, it is present only for a short time early in life and during pregnancy, and is not known to have any function.

IV THE POSTERIOR LOBE 

 
Two hormones are secreted by the posterior lobe. One of these is the antidiuretic hormone (ADH), vasopressin. Vasopressin stimulates the kidney tubules to absorb water from the filtered plasma that passes through the kidneys and thus controls the amount of urine secreted by the kidneys. The other posterior pituitary hormone is oxytocin, which causes the contraction of the smooth muscles in the uterus, intestines, and blood arterioles. Oxytocin stimulates the contractions of the uterine muscles during the final stage of pregnancy to stimulate the expulsion of the fetus, and it also stimulates the ejection, or let-down, of milk from the mammary gland following pregnancy. Synthesized in 1953, oxytocin was the first pituitary hormone to be produced artificially. Vasopressin was synthesized in 1956.

V PITUITARY DISTURBANCES 

 
Pituitary functioning may be disturbed by such conditions as tumors, blood poisoning, blood clots, and certain infectious diseases. Conditions resulting from a decrease in anterior-lobe secretion include dwarfism, acromicria, Simmonds's disease, and Fröhlich's syndrome. Dwarfism occurs when anterior pituitary deficiencies occur during childhood; acromicria, in which the bones of the extremities are small and delicate, results when the deficiency occurs after puberty. Simmonds's disease, which is caused by extensive damage to the anterior pituitary, is characterized by premature aging, loss of hair and teeth, anemia, and emaciation; it can be fatal. Fröhlich's syndrome, also called adiposogenital dystrophy, is caused by both anterior pituitary deficiency and a lesion of the posterior lobe or hypothalamus. The result is obesity, dwarfism, and retarded sexual development. Glands under the influence of anterior pituitary hormones are also affected by anterior pituitary deficiency.

Oversecretion of one of the anterior pituitary hormones, somatotropin, results in a progressive chronic disease called acromegaly, which is characterized by enlargement of some parts of the body. Posterior-lobe deficiency results in diabetes insipidus.

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This page was last updated on 11/29/08 03:35:56 PM.

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