Endocrine System

The pituitary gland has long been called "the master gland," because it secretes multiple hormones which, in turn, trigger the release of other hormones from other endocrine sites. The pituitary is roughly situated behind the nose and is anatomically separated into two distinct lobes, the anterior pituitary (AP) and the posterior pituitary (PP). The entire pituitary hangs by a thin piece of tissue, called the pituitary stalk, beneath the hypothalamus in the brain. The AP and PP are sometimes called the adenohypophysis and neurohypophysis, respectively.

The PP secretes two hormones, oxytocin and antidiuretic hormone (ADH), under direction from the hypothalamus. Direct innervation of the PP occurs from regions of the hypothalamus called the supraoptic and paraventricular nuclei. Although the PP secretes its hormones into the bloodstream through blood vessels that supply it, it is regulated in a neuroendocrine fashion. The AP, on the other hand, receives hormonal signals from the blood supply within the pituitary stalk.

AP cells are categorized according to the hormones that they secrete. The hormone-producing cells of the AP include: somatotrophs, corticotrophs, thyrotrophs, lactotrophs, and gonadotrophs. Somatotrophs secrete growth hormone; corticotrophs secrete adrenocorticotropic hormone (ACTH); thyrotrophs secrete thyroid stimulating hormone (TSH); lactotrophs secrete prolactin; and gonadotrophs secrete LH and follicle stimulatory hormone (FSH). Each of these hormones sequentially signals a response at a target site. While ACTH, TSH, LH, and FSH primarily stimulate other major endocrine glands, growth hormone and prolactin primarily coordinate an endocrine response directly on bones and mammary tissue, respectively.

The human endocrine system. Illustration by Argosy. The Gale Group.