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simple, usually unbranched tubular glands of the skin in man and mammalian animals (except moles, pangolins, sloths, some pinnipeds, whales, and Sirenia) that produce and excrete sweat.
The total number of sweat glands in man is from 2 to 5 million. Their number and size vary in different parts of the body. There are many sweat glands in the skin of the finger and toe pads, palms and soles, and axial and inguinal folds (average, from 150 to 339; on the skin of the palm up to 1,000 sweat glands per sq cm). The sweat glands of mammals are derivatives of multicellular glands of the integuments of amphibians, since both kinds are lined with smooth musculature of ectodermal origin— myoepithelial cells. The saclike untwisted shape of sweat glands is characteristic of primitive mammals, such as marsupials.
The sweat glands consist of an unbranched excretory duct and an acinus, which, like the beginning of the excretory duct, is rolled in a ball located on the boundary between the reticular layer of the skin and the subcutaneous tissue. The excretory ducts open through apertures, or sweat pores, to the surface of the skin, and on hairy parts of the body usually into the hair follicles. The secretory section consists of a single layer of prismatic cells with tiny vacuoles, droplets of fat, and granules of glycogen and pigment. The appended myoepithelial cells, which excrete sweat by their contractions, are located on the basal membrane.
Sweat glands are classified according to type of secretion as ordinary merocrine glands, which are most highly developed among humans and primates, and specific apocrine glands of the majority of mammals. The special varieties of sweat glands of the eyelids and of the ears produce cerumen. In old age the number of sweat glands in man diminishes, and their secretory section is curtailed. The sweat glands are innervated by sympathetic nerves, whose endings secrete acetylcholine when they are stimulated— that is, they are cholinergic, as are the parasympathetic nerves. By producing a large quantity of sweat, the sweat glands regulate heat emission; excrete the products of nitrogen metabolism and salts of alkaline metals, mainly sodium chloride; and moisten the surface of the skin.
REFERENCEShmal’gauzen, I. I. Osnovy sravnitel’noi anatomii pozvonochnykh zhivotnykh, 4th ed. Moscow, 1947.
Gistologiia, 2nd ed. Edited by V. G. Eliseev et al. Moscow, 1972.
Sokolov, V. E. Kozhnyi pokrov mlekopitaiushchikh. Moscow, 1973.