Glands are broadly classified as endocrine or exocrine. They originate as invaginations of a surface epithelium. In exocrine29 (EC-so-crin) glands, they usually maintain their contact with the surface by way of a duct, an epithelial tube that conveys their secretion to the surface. The secretion may be released to the body surface, as in the case of sweat, mammary, and tear glands, but more often it is released into the cavity (lumen) of another organ such as the mouth or intestine. Endocrine30 (EN-doe-crin) glands lose their contact with the surface and have no ducts. They do, however, have a high density of blood capillaries and secrete their products directly into the blood. The secretions of endocrine glands, called hormones, function as chemical messengers to stimulate cells elsewhere in the body. Endocrine glands are the subject of chapter 17 and are not further considered here.
The exocrine-endocrine distinction is not always clear. The liver is an exocrine gland that secretes one of its
29exo = out + crin = to separate, secrete
30endo = in, into
Saladin: Anatomy & I 5. Histology I Text I I © The McGraw-Hill
Physiology: The Unity of Companies, 2003 Form and Function, Third Edition
180 Part One Organization of the Body products, bile, through a system of ducts but secretes hormones, albumin, and other products directly into the bloodstream. Several glands have both exocrine and endocrine components, such as the pancreas, testis, ovary, and kidney. And nearly all of the viscera have at least some cells that secrete hormones, even though most of these organs are not usually thought of as glands (for example, the brain and heart).
Unicellular glands are exocrine cells found in an epithelium that is predominantly nonsecretory. For example, the respiratory tract, which is lined mainly by ciliated cells, also has a liberal scattering of nonciliated, mucus-secreting goblet cells (see figs. 5.6 and 5.7).
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