The pituitary gland, also called hypophysis, is the main gland of the endocrine system. It plays an important role in hormonal regulation in human beings and vertebral animals, secreting hormones that stimulate other endocrine glands. It is located at the base of the brain, below the hypothalamus, as it lies in the sella turcica of the sphenoid bone. The pituitary is bean-shaped and measures 8 mm in length and 6 mm in width, with an average weight of 550 grams in an adult individual of a human being. This gland is connected to the base of the brain by the pituitary stalk or infundibular stem, an outgrowth of the floor of the third cerebral ventricle.
Anatomical Description and Origin
The hypophysis is connected to the hypothalamus through the pituitary stalk. The gland is composed of 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, on the other hand, 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.
The posterior lobe of the pituitary gland is innervated by a large number of nerve fibers that enter it from the hypothalamus through the stalk and along the walls of the hypophyseal arteries that arise from the internal carotid. These nerve fibers are arranged in two tracts or bundles; one arising and descending from the paraventricular nucleus and the other from the supraoptic nucleus of hypothalamus. Once they make their way into the stalk, they get together to form the supraoptico-hypophysial tract. The anterior lobe, on the other hand, receives nerve fibers from the tuberal nuclei through the tuberoinfundibular tract.
Function
The anterior lobe of the pituitary gland plays a glandular-secretory role. Growth, reproduction, and basal, fat, protein, carbohydrate and mineral metabolism depend upon its normal functioning. Thus, the anterior lobe secretes seven hormones: growth hormone (somatotropin), thyrotropic hormone, follicle-stimulating hormone, luteinizing hormone (lutropin), luteotropic hormone, lactogenic hormone (prolactin), and adrenocorticotropic hormone (ACTH). All these hormones are protein in nature and can be obtained in pure form. 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.
The posterior lobe of the pituitary gland takes part in the regulation of blood pressure and urine output, through the hormone vasopressin. It also takes part in the 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. Meanwhile, 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.
Blood Supply
The pituitary gland is supplied with oxygenated blood by the superior and inferior hypophysial artery, which arise from the internal carotid. These two arteries enter the median emminence of hypothalamus, forming the superficial external plexus, which in turn gives rise to a network of capillaries. Meanwhile, the gland is drained by the hypophysial portal system of capillaries and venules, which empty deoxygenated blood into the cavernous sinus.
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| Above, schematic picture showing the pituitary gland with its arterial blood supply and the hypophysial portal system that end up in the veins to the dural cavernous sinuses. The nerve tracts from hypothalamic nuclei can also be seen. |
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| The hypophysis gross anatomy, showing the three lobes and the pituitary stalk. |