The layer of cells that covers the entire surface of the body and lines most of the hollow structures within it. Epithelial cells vary in shape according to their function. There are three basic shapes of cell: squamous (thin and flat), cuboidal, and columnar. Most internal organs lined with epithelium are covered with only one layer of cells, but the skin, which is subjected to more trauma, consists of many layers.
Epithelium is the cellular layer which forms the epidermis on the skin, covers the inner surface of the bowels, and forms the lining of ducts and hollow organs, like the bladder. It consists of one or more layers of cells which adhere to one another, and is one of the simplest tissues of the body. It is of several forms: for example, the epidermis is formed of scaly epithelium, the cells being in several layers and more or less ?attened. (See SKIN.) The bowels are lined by a single layer of columnar epithelium, the cells being long and narrow in shape. The air passages are lined by ciliated epithelium: that is to say, each cell is provided with ?agellae (lashes) which drive the ?uid upon the surface of the passages gradually upwards.
n. the tissue that covers the external surface of the body and lines hollow structures (except blood and lymphatic vessels). It is derived from embryonic ectoderm and endoderm. Epithelial cells may be flat and scalelike (squamous), cuboidal, or columnar. The latter may bear cilia or brush borders or secrete mucus or other substances (see goblet cell). The cells rest on a common basement membrane, which separates epithelium from underlying *connective tissue. Epithelium may be either simple, consisting of a single layer of cells; stratified, consisting of several layers; or pseudostratified, in which the cells appear to be arranged in layers but in fact share a common basement membrane (see illustration). See also endothelium; mesothelium. —epithelial adj.
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