The hypophysis, also known as the pituitary gland, lies at the base of the brain in the sella turcica and is connected to the hypothalamus by the hypophyseal stalk. The hypophysis has two distinct portions: (1) the adenohypophysis (anterior pituitary) and (2) the neurohypophysis (posterior pituitary). The adenohypophysis arises from upward growth of pharyngeal epithelium in the embryo, whereas the neurohypophysis arises from the downward growth of the hypothalamus in the embryo.
Almost all hormonal secretion from the hypophysis is controlled by the hypothalamus. Neuro-hypophyseal hormones are formed in the hypothalamus and travel down nerve fibers to the neurohypophysis, where they are stored and then released into the circulation in response to feedback mechanisms. Adenohypophyseal hormone secretion is controlled by releasing and inhibiting factors that are secreted by the hypothalamus and carried to the adenohypophysis by the hypothalamic-hypophyseal portal vessels. Hypothalamic releasing and inhibiting factors identified thus far include (1) thyrotropin-releasing hormone (TRH); (2) corticotropin-releasing hormone (CRH); (3) gonadotropin-releasing hormone (GnRH), also known as luteinizing hormone-releasing hormone (LHRH) and follicle-stimulating hormone-releasing factor; (4) growth hormone-releasing hormone (GHRH); (5) growth hormone-inhibiting hormone (GHIH); and (6) prolactin inhibitory hormone (PIH). A releasing factor for melanocyte-stimulating hormone also is believed to exist. The releasing factors either stimulate or inhibit the adenohypophysis in the release of its hormones.
The adenohypophysis consists of three major cell types: (1) acidophils, (2) basophils, and (3) chromophobes. The acidophils secrete growth hormone (GH), also called somatotropic hormone (STH, SH) or somatotropin, and prolactin (HPRL), also known as luteotropic hormone (LTH), lactogenic hormone, or lactogen. The basophils secrete adrenocorticotropic hormone (ACTH), also known as adrenocorticotropin and corticotropin; thyroid-stimulating hormone (TSH), also known as thyrotropin; follicle-stimulating hormone (FSH); luteinizing hormone (LH), also known as interstitial cell-stimulating hormone (ICSH); and melanocyte-stimulating hormone (MSH). The chromophobes, which constitute about half of the adenohypophyseal cells, are resting cells capable of transformation to either acidophils or basophils.
The hormones stored and released by the neurohypophysis include antidiuretic hormone (ADH), also known as vasopressin, and oxytocin. Radio-immunoassays are used to determine the blood levels of the hypophyseal hormones.