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pl. epithelia [epi- + Gr. thēlē, nipple, teat + -ium (2)]
The layer of avascular cells forming the epidermis of the skin and the surface layer of mucous and serous membranes. The cells rest on a basement membrane and lie close to one another with little intercellular material between them. The epithelium may be simple (consisting of a single layer) or stratified (consisting of several layers). Cells making up the epithelium may be flat (squamous), cuboidal, or cylindrical (columnar). Modified forms of epithelium include ciliated, pseudostratified, glandular, and neuroepithelium. The epithelium may include goblet cells, which secrete mucus. Stratified squamous epithelium may be keratinized for a protective function or abnormally keratinized in response to physical forces or disease. Squamous epithelium is classified as endothelium, which lines the blood vessels and the heart, and mesothelium, which lines the serous cavities. Epithelium serves the general functions of protection, absorption, and secretion, and specialized functions such as movement of substances through ducts, production of germ cells, and reception of stimuli. Its ability to regenerate is excellent: it may replace itself as often as every 24 hr.
GLANDULAR EPITHELIUM (Orig. mag. ×430); SEE: skin
epithelial (ep″i-thē′′lē-ăl ), adj.