Definition: an unpaired compound gland suspended from the base of the hypothalamus by a short extension of the infundibulum, the infundibular or pituitary stalk. The hypophysis consists of two major divisions. The first, the neurohypophysis, comprises the infundibulum and its bulbous termination, the neural part or infundibular process (posterior lobe), which is composed of neuroglialike pituicytes, blood vessels, and unmyelinated nerve fibers of the hypothalamohypophysial tract. The cell bodies of these axons reside in the supraoptic and paraventricular nuclei of the hypothalamus. These fibers convey to the lobe for storage and release the neurosecretory hormones oxytocin and antidiuretic hormone. The second division, the adenohypophysis, comprises the larger distal part, a sleevelike extension of this lobe (infundibular part) that invests the infundibular stalk, and a thin intermediate part (poorly developed in humans) between the anterior and posterior lobes; the anterior lobe consists of cords of cells of several different types interspersed on the secondary capillary bed of the hypothalamohypophysial portal system. Secretion of somatotropins, prolactin, thyroid-stimulating hormone, gonadotropins, adrenal corticotropin, and other related peptides in the adenohypophysis is regulated by releasing and inhibiting factors elaborated by neurons in the hypothalamus that are taken up by a primary plexus of capillaries in the median eminence and transported through portal vessels in the infundibular part and infundibular stem to a secondary plexus of capillaries in the distal part.
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