small oval endocrine gland that lies at the base of the brain. It is sometimes called the master gland of the body because all the other endocrine glands depend on its secretions for stimulation (see endocrine system).
Physiologically, the pituitary is divided into two distinct lobes that arise from different embryological sources. The anterior lobe, or adenohypophysis, grows upward from the pharyngeal tissue at the roof of the mouth. An intermediate lobe also originates in the pharynx, but in humans it is greatly reduced in structure and function. The posterior lobe, or neurohypophysis, grows downward from neural tissue. It is structurally continuous with the hypothalamus of the brain, to which it remains attached by the hypophyseal, or pituitary, stalk. The hypothalamus controls almost all secretions of the pituitary. The posterior lobe is controlled by nerve fibers that originate in hypothalamic neurons and the anterior lobe by substances that are transported from the hypothalamus by tiny blood vessels.
The tissues in the anterior lobe consist of extensive vascular areas interspersed among glandular cells that secrete at least six different hormones. It was formerly believed that a master molecule was stimulated by various enzymes to produce these hormones, but present evidence indicates that each is individually synthesized, probably by a specific type of glandular cell. Three such types of cells exist in the anterior pituitary gland: acidophils, basophils, and chromophobes. The growth hormone, thought to be synthesized by certain acidophils, stimulates all the tissues in the body to grow by effecting protein formation.
The remaining five important hormones influence body functions by stimulating target organs. Adrenocorticotropic hormone (ACTH) controls the secretion of steroid hormones by the adrenal cortex, which affects glucose, protein, and fat metabolism; thyrotropin controls the rate of thyroxine synthesis by the thyroid gland, which is the principal regulator of body metabolic rate; prolactin, which regulates the formation of milk after the birth of an infant; and three separate gonadotropic hormones (follicle-stimulating hormone, luteinizing hormone, and luteotropic hormone) control the growth and reproductive activity of the gonads.
The release of each of the hormones from the anterior lobe is controlled by a specific substance secreted by nerve cells in the hypothalamus. These substances, called releasing factors, are transmitted by nerve fibers to tiny capillaries in the hypophyseal stalk. They move through blood vessels to the anterior lobe, where each releasing factor is responsible for the release of a specific pituitary hormone.
The two hormones that are produced by the posterior lobe are synthesized by nerve cells in the hypothalamus. They are transported by nerve fibers to nerve endings in the posterior lobe, where they are released. The hormones are antidiuretic hormone (ADH or vasopressin), which alters the permeability of the kidney tubules, permitting more water to be retained by the body; and oxytocin, which aids in the release of milk from mammary glands and causes uterine contractions. The only hormone that is synthesized by the intermediate lobe is the melanocyte-stimulating hormone, which appears to control skin pigmentation.
Oversecretion of the pituitary hormone human growth hormone can cause gigantism if it occurs before growth of the long bones is complete, or acromegaly if it begins during adulthood. Undersecretion of human growth hormone can lead to dwarfism if experienced during childhood, and decreased endocrine function accompanied by lethargy and loss of sexual capacity in the adult.
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