Pituitary Gland

The pituitary gland, the hypophysis, is a small structure at the base of the brain composed of two glands the adenohypophysis and neurohypophysis that control homeostasis, growth, and reproduction. Hormones of the neurohypophysis, or posterior pituitary gland, vasopres-sin, vasotocin and oxytocin, are produced as prohormones in the hypothalamus and are transported via neural axons to the neurohypophysis. There they are stored, processed, and released into the circulation. Vasopressin or antidiu-retic hormone (ADH) stimulates blood vessel constriction and water resorption by kidneys and enhances cortico-tropin (ACTH) secretion from the anterior pituitary. Vasotocin and vasopressin are structurally and functionally similar. Oxytocin acts on the uterus to stimulate contractions and mammary glands to induce milk ejection.

The adenohypophysis, or anterior pituitary gland, produces and secretes ACTH, GH, LH, FSH, TSH, prolactin (PRL), melanocyte-stimulating hormone (MSH), ß-endorphin, and ß-lipoprotein. ACTH, MSH, and ß-endorphin are cleavage products of POMC gene regulated by CRH. ACTH stimulates glucocorticoid synthesis in the adrenal cortex in response to stress. GH has actions in growth and development, immune development, reproduction, and lactation. GH is under dual hypothalamic regulation, GRH and SRIH. LH and FSH are regulated by GnRH. LH is the regulator of testosterone production by Leydig cells in testes and the stimulus for ovulation and maintenance of corpora lutea in ovaries. Ovarian follicle recruitment and development and testic-ular Sertoli cell function are dependent upon FSH. TSH regulates synthesis and release of thyroxine (T4) from the thyroid. Thyroxine is converted to biologically active triiodothyronine (T3), which regulates oxidation of fats, proteins, and carbohydrates in the liver, kidneys, heart, and muscle, and thus regulates basal metabolism. TSH, LH, and FSH are dimeric glycoproteins sharing a common a subunit. PRL has roles in maintenance of corpora lutea and lactation. Consensus PRL-release inhibiting factor is dopamine but no specific PRL-releasing factor has been identified.

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