Pituitary Gland and Hormones
The pituitary gland is sometimes called the "master" gland of the endocrine system, because it controls the functions of many other endocrine glands. It is a small, oval gland, no larger than a pea, and is located at the base of the brain. It is attached to the hypothalumus (a part of the brain that affects the pituitary gland) by nerve fibers.
Anatomically, the pituitary gland consists of three sections: the anterior lobe, the intermediate lobe and the posterior lobe. The anterior and posterior lobes are derived from different embryological sources. The anterior lobe, or adenohypophysis, grows upward from the pharyngeal tissue at the roof of the mouth. The posterior lobe, or neurohypophysis, grows downward from neural tissue. It is structurally continuous with the hypothalamus, to which it remains attached by the hypophyseal, or pituitary, stalk. The intermediate lobe also originates in the pharynx, like the anterior lobe, but in humans it is greatly reduced in structure and function. The hypothalamus controls almost all secretions of the pituitary gland. 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 a number of different hormones. It was formerly believed that various enzymes stimulated a master molecule, which then produced these hormones. Present evidence indicates that each hormone is individually synthesized by a specific type of glandular cell. Three types of such cells exist in the anterior pituitary gland: acidophils, basophils, and chromophobes.
The anterior lobe secretes important hormones that influence bodily functions by stimulating target organs. Adrenocorticotropic hormone (ACTH) is a peptide of 39 amino acids and controls the secretion of steroid hormones by the adrenal cortex. These affects glucose, protein, and fat metabolism. ACTH also stimulates the production of androgens, like testosterone, and in the foetus it stimulates the adrenal cortex to synthesize a precurser to estrogen called dehydroepiandrosterone sulfate (DHEA-S), which helps prepare the mother for giving birth. Another important hormone produced by the anterior pituitary lobe is thyroid stimulating hormone (TSH), also known as thyrotropin. This controls the rate of thyroxine synthesis by the thyroid, which is the principal regulator of body metabolic rate.
Human growth hormone (GH), or somatotropin, is a protein of 191 amino acids and GH-secreting cells in the anterior lobe are stimulated to synthesize and release GH by the intermittent arrival of growth hormone releasing hormone (GHRH) from the hypothalamus. GH promotes bodily growth by binding to receptors on the surface of liver cells, stimulating them to release growth factors promoting bone elongation. Oversecretion of GH can cause gigantism if it occurs before growth of the long bones is complete, or acromegaly if it begins during adulthood. Undersecretion can lead to dwarfism if experience during childhood, and decreased endocrine function accompanied by lethargy and loss of sexual capacity in the adult.
Other hormones secreted by the anterior lobe include prolactin, which regulates the formation of milk after the birth of an infant and also three separate gonadotropic hormones, follicle-stimulating hormone (FSH), luteinizing hormone (LH), and luteotropic hormone which 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 one is responsible for the release of a specific pituitary hormone.
The posterior lobe releases two hormones into the body's circulation, both of which 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), a very small peptide of nine amino acids, which acts on the collecting ducts of the kidney to facilitate the reabsorption of water into the blood. This acts to reduce the volume of urine formed, hence it is named antidiuretic hormone. The other hormone secreted by the posterior lobe is oxytocin, also a peptide of none amino acids, which aids in the release of milk from mammary glands and causes uterine contractions. Oxytocin is often given to mothers to hasten birth. The only hormone that is synthesized by the intermediate lobe is the melanocyte-stimulating hormone, which appears to control skin pigmentation.
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