Post your articles online and promote your website for free! Boost your sites ranking by linking it from the ArticleZap database! Free Article directory and instant translation publishing!
pituitary gland, also called HYP0PHYSIS, one of the endocrine (ductless) glands that secrete their hormones directly into the bloodstream. The term hypophysis (from the Greek, “lying under”) describes the gland’s position on the lower surface of the vertebrate brain. The pituitary plays a vital role in the animal’s physiology; and because its effects are mostly mediated through its control over other endocrine glands, it is frequently called the “master gland.”
In the adult human, the pituitary gland weighs about 0.5 gram (0.02 ounce) and measures about 1 centimetre (0.4 inch) long, 1.5 centimetres wide, and 0.6 centimetre deep, That of the whale weighs from 10-60 grams; of cattle, 2.5 grams; of the dog, 0.065 gram; and the cat, 0.009 gram. The gland is situated in a depression called the sella turcica (Turkish saddle) on the sphenoid bone—deep in some species, including man, and shallow in others. The gland consists of two anatomical divisions: (1) the neurohypophysis, derived embryobogically from the floor of the diencephabon, and (2) the adenohypophysis, derived from an upward outpouching of the roof of the mouth (Rathke’s pouch). The neurohypophysis consists of three parts: the median eminence of the tuber cinereum, the infundibular stem, and the infundibular process, or neural lobe. In most species the posterior portion of the developing adenohypophysis becomes so closely applied to the neural lobe that the two cannot be separated; together they form the posterior lobe.
The portion of the posterior lobe derived from the adenohypophysis is known as the pars intermedia (intermediate lobe). In certain animals such as the cetaceans, armadillos, beavers, and birds, the intermediate lobe is absent; and the anterior and neural lobes are separated by a thin layer of connective tissue. The anterior lobe consists of branching cords of cells supported by a reticulum of connective tissue and separated by vascular channels (sinusoids). The cells are classified as chromophobes and chromophils, based on their staining properties. About 75 percent of the latter contain granules stainable with acidophilic dyes and are known as acidophils, or alpha cells.
The other 25 percent stain with basophilic dyes and are known as basophils, or beta cells.
Sufficient specific cell types have been demonstrated in the human adenohypophysis to indicate that each cell type secretes only one (or possibly two closely related) pituitary hormones. The acidophil appears to be the production site of prolactin (luteotropic hormone, LTI-I) and somatotropic hormone (STH or growth hormone, OH). Subtypes of basophil cells seem to secrete the following: thyrotropic hormone (thyroid-stimulating hormone, TsH), adrenocorticotropic hormone (Ac-ru), follicle-stimulating hormone (F5H), luteinizing hormone (LH, or interstitial cell stimulating hormone, ICSH), and melanocytestimulating hormone (MSH), intermedin. In animals in which there is an anatomically distinct intermediate lobe, usu is found at that site. The preceding anterior pituitary hormones, with the exception of STH and MSH, exert their primary action on other endocrine glands (“target” glands or cells), causing them to secrete their own hormones, (e.g., rsH causes the thyroid gland to secrete thyroid hormone). Blood levels of the latter, in turn, check the stimulating action of the pituitary;
-
TSH-----> TH ----- TSH
TH-----> etc.,
in a cycle known as “feedback control.”
Two hormones have been extracted from the posterior pituitary: oxytocin (pitocin) and vasopressin or antidiuretic hormone (ADH, or pitressin). They are structurally similar, each containing eight amino acids but in different sequences. Vasopressin causes an elevation of blood pressure via direct action on the muscles of the blood vessels and exerts a powerful antidiuretic action because of its effect on the kidney tubule cells in which it increases reabsorption of water. Oxytocin causes contraction of the uterus, a fall of blood pressure in birds. and secretion of milk from the breasts of lacating mammals during suckling. Vasopressin appears to exert considerable oxytocin effect; and oxitocin has, but to a much lesser degree, some antidiuretic potency.
