PITUITARY GLAND

also called hypophysis one of the endocrine (ductless) glands that secrete their hormones directly into the bloodstream. The term hypophysis (from the Greek, lying under) refers to the gland's position on the underside of the vertebrate brain. Until the late 19th century the human pituitary was thought to be a vestigial organ, but it is now known to play a major part in the regulation of endocrine functions. The pituitary is divided into two lobes: the anterior, or adenohypophysis, which is derived from an upward outpouching of the roof of the mouth (Rathke's pouch), and the posterior, or neurohypophysis, which is derived from embryonic nerve tissues. Most of the pituitary hormones are secreted by the anterior lobe and cause the production or release of hormone from other endocrine glands. The thyroid-stimulating hormone (TSH) stimulates the growth of the thyroid gland and release of its hormone; the adrenocorticotropic hormone (ACTH) regulates the endocrine activities of the cortex of the adrenal glands, which produces cortisol; follicle-stimulating hormone (FSH) promotes secretion of the female hormone estrogen and the development of egg and sperm cells; luteinizing hormone releases estrogen, progestin, and the male hormone testosterone; and growth hormone, or somatotropin, stimulates a variety of systems involved in the growth of the individual. The two other anterior pituitary hormones do not act on endocrine glands but directly affect specific tissues. They are prolactin, which causes breast development and milk production, and melanocyte-stimulating hormone (MSH), which stimulates pigment cells. The parent hormone of MSH, lipoprotein, also gives rise to chemical compounds, called enkephalins and endorphins, that have an effect on brain cells similar to that of opiates such as morphine. The posterior lobe, or neurohypophysis, is connected to the anterior lobe by a portal vein, through which the hormones that control pituitary function are transmitted. These hormones originate in the hypothalamus of the brain and are stored in the neurohypophysis until needed. The neurohypophysis also releases two hormones that act elsewhere in the body: oxytocin, which causes contraction of the uterus and milk secretion in female mammals and lowers blood pressure in birds; and vasopressin, also called antidiuretic hormone (ADH), which raises blood pressure by causing the contraction of blood vessels and increases reabsorption of water from the kidneys.