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The pituitary (hypophysis) is a small gland (350–900 mg) located in the sella turcica, a bony compartment in the base of the skull. It is composed of an anterior lobe (adenohypophysis), which comprises about 75% of the gland; a posterior lobe (neurohypophysis), which comprises about 25% of the gland; and a vestigial intermediate lobe (Figure 57-1).

Figure 57–1.

Principal hormones of the pituitary and hypothalamus, their target organs, and their effects. Hypothalamic neurosecretory cells secreting ADH and oxytocin have direct axonal connections with the posterior pituitary. Hypothalamic cells secreting releasing and inhibiting hormones that control pituicytes in the anterior lobe exert their controlling influence via the portal venous system in the pituitary stalk. (STH, somatotropic hormone; LH, luteinizing hormone; FSH, follicle-stimulating hormone; ACTH, adrenocorticotropic hormone (corticotropin); TSH, thyroid-stimulating hormone (thyrotropin); ADH, antidiuretic hormone.)

Histologically, the anterior pituitary is composed of small round cells in nests and cords separated by a rich vascular network. The cells have variably staining cytoplasm on routine sections and were at one time called acidophils, basophils, and chromophobes based on their staining characteristics. They are now classified according to the specific hormones they produce (Table 57-1) as identified by immunohistochemical methods. About 15–20% of the cells in the anterior pituitary are nonreactive to immunohistochemical tests and are classified as nonsecretory cells.

Table 57–1. Cell Types and Hormones of the Pituitary.

The posterior lobe is composed of a mass of nerve fibers with supporting glial cells. These unmyelinated nerves are the axons of hypothalamic neurons. As shown by electron microscopy, they contain membrane-bound secretory granules (composed either of antidiuretic hormone [ADH] ...

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