human growth hormone (HGH): see growth hormone.

growth hormone or somatotropin, glycoprotein hormone released by the anterior pituitary gland that is necessary for normal skeletal growth in humans (see protein). Evidence suggests that the secretion of human growth hormone (HGH) is regulated by the release of certain peptides by the hypothalamus of the brain. One such substance, called somatostatin, has been shown to inhibit the secretion of HGH. HGH is known to act upon many aspects of cellular metabolism, but its most obvious effect is the stimulation of the growth of cartilage and bone in children.

See also auxins (plant growth hormones).

Role in Dwarfism and Gigantism

A deficiency of growth hormone secretion before puberty (by the end of which the synthesis of new bone tissue is complete) results in pituitary dwarfism. Pituitary dwarfs, who can be as little as 3 to 4 ft (91-122 cm) tall, are generally well proportioned except for the head, which may be relatively large when compared to the body (this relationship of head to body is similar to that of normal children). Unlike cretins, whose dwarfism is caused by a deficiency of thyroxine, pituitary dwarfs are not mentally retarded; they are often sexually immature. They can be treated by injections of synthetic growth hormone, either somatrem or somatropin, which are produced by genetically engineered bacteria.

An excess of growth hormone in children results in gigantism; these children grow to be over 7 ft (213 cm) in height and have disproportionately long limbs. Excess growth hormone produced after puberty has little effect on the growth of the skeleton, but it results in a disease affecting terminal skeletal structures known as acromegaly.

Other Medical Uses

HGH has been used with some success to combat the weight loss and general wasting characteristic of AIDS and cancer. It is used illegally by bodybuilders and athletes to increase muscle mass. Controversy surrounds its use in normal children who simply want to be taller. In addition, a 1990 medical study that reported the reversal of many of the physiological effects of aging with regular injections of HGH has created a lucrative black market for it and has prompted funding of further trials. There has been no conclusive evidence, however, to support the use of HGH as an anti-aging treatment, and it can cause serious side effects, including diabetes, in older adults.

or somatotropin

Peptide hormone secreted by the anterior lobe of the pituitary gland. It promotes growth of bone and other body tissues by stimulating protein synthesis and fat breakdown (for energy). Excessive production causes gigantism, acromegaly, or other malformations; deficient production results in dwarfism, dramatically relieved if GH is given before puberty. Genetic engineering techniques now permit large-scale production of adequate amounts of GH for that purpose.

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In a cross section of the stem of a woody plant, the amount of wood added during a single growth period. In temperate regions this period is usually one year, in which case the growth ring may be called an annual ring. In tropical regions growth rings may not be discernible or are not annual. Even in temperate regions growth rings are occasionally missing; and sometimes a second, or “false,” ring may be deposited during a single year (e.g., following defoliation by insects). Nevertheless, annual rings have been used in dating ancient wooden structures, especially those of American Indians in the dry U.S. Southwest. Changes in ring width are a source of information about ancient climates.

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Process of growing a crystal of a particular orientation on top of another crystal. If both crystals are of the same material, the process is known as homoepitaxy; if the materials are different, it is known as heteroepitaxy. Common types of epitaxy include vapour phase, liquid phase, and solid phase, according to the source of the atoms being arranged on the substrate. Epitaxy is most frequently employed in the production of semiconductor wafers for use in the creation of integrated circuits.

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Process by which a nation's wealth increases over time. The most widely used measure of economic growth is the real rate of growth in a country's total output of goods and services (gauged by the gross domestic product adjusted for inflation, or “real GDP”). Other measures (e.g., national income per capita, consumption per capita) are also used. The rate of economic growth is influenced by natural resources, human resources, capital resources, and technological development in the economy along with institutional structure and stability. Other factors include the level of world economic activity and the terms of trade. Seealso economic development.

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