The Pituitary Gland

The pituitary gland (also called the hypophysis) is a small, pea-sized endocrine gland lying in a bony depression at the base of the skull called the sella turcica, connected to the overlying hypothalamus by the narrow infundibulum (pituitary stalk). Despite its small size, the pituitary is often described as the 'master gland' of the endocrine system, because several of the hormones it secretes act not on ordinary body tissues directly but on other endocrine glands, switching their own hormone output up or down; the pituitary itself, however, is not truly independent, since its own activity is closely governed moment to moment by the hypothalamus above it, as described in the previous section.

Structurally the pituitary is made up of two principal parts of quite different embryonic origin, function and mode of hypothalamic control. The anterior pituitary (adenohypophysis) develops from an upward outgrowth of the embryonic pharynx (Rathke's pouch) and is a genuine glandular tissue in its own right, containing several distinct populations of hormone-secreting cells — somatotrophs (growth hormone), thyrotrophs (TSH), corticotrophs (ACTH), gonadotrophs (FSH and LH) and lactotrophs (prolactin) — each stimulated or inhibited by its own matching hypothalamic releasing/inhibiting hormone arriving via the hypophyseal portal blood vessels. The posterior pituitary (neurohypophysis), by contrast, is developmentally an outgrowth of the brain itself, made of neural tissue rather than true gland tissue, and consists chiefly of the axon terminals of hypothalamic neurosecretory cells together with specialised supporting (pituicyte) cells and a dense capillary network; as explained in the previous section, it stores and releases, rather than synthesises, the two hormones oxytocin and vasopressin. Between these two principal lobes lies a thin, often largely rudimentary pars intermedia, which in the adult human is mostly fused with the anterior lobe but still contributes melanocyte-stimulating hormone (MSH).

The anterior pituitary secretes six hormones altogether. Growth hormone (GH, or somatotropin) is secreted in the greatest quantity of any anterior pituitary hormone; it acts on bone, muscle and the liver (partly through an intermediate liver-derived signal, insulin-like growth factor-1) to stimulate overall body growth, protein synthesis and cell division, and its deficiency or excess in childhood, before the epiphyseal growth plates of the long bones have fused, is directly responsible for the growth disorders dwarfism and gigantism respectively, taken up later in the chapter. Thyroid-stimulating hormone (TSH) acts on the thyroid gland, stimulating both the synthesis and the release of the thyroid hormones T3 and T4. Adrenocorticotropic hormone (ACTH) acts on the adrenal cortex, stimulating it to secrete glucocorticoids, chiefly cortisol. Follicle-stimulating hormone (FSH) and luteinising hormone (LH), together called the gonadotropins because they act on the gonads, stimulate spermatogenesis (via the Sertoli cells) and testosterone secretion (via the Leydig cells) in the male, and stimulate follicle growth and, in the case of LH, ovulation and corpus luteum formation, in the female. Prolactin (PRL) acts on the mammary glands, stimulating milk production after childbirth. …