epithelium

epithelium , sheet of tissue that covers or lines the external and internal body surfaces. The epithelium is closely packed, has little intercellular material, and is lacking in blood vessels. There are three characteristic types of epithelial cells: squamous, cuboidal, and columnar. Squamous epithelial cells are flat and often overlapping; they compose the outer layer of skin (epidermis) and line certain internal cavities, e.g., the mouth. Cuboidal epithelial cells are rounded and elastic and line such structures as the urinary bladder, where, by stretching and becoming flatter, they increase the organ's capacity to hold fluid. The cells of the columnar epithelium are long and thin; they are found as a single layer of secretory and absorptive cells in the gastrointestinal tract, and they form the ciliated lining of the respiratory tract. Embryologically, epithelium may be derived from any of the three germ layers, i.e., ectoderm, mesoderm, and endoderm, and may be classified accordingly as epithelium proper, mesothelium, and endothelium.

epithelium Epithelia are tissues lining the outer surface of the body (skin), or the inner surface of organs which have a direct connection to one of the body's orifices. The latter group includes tissues lining the airways; the alimentary canal and its associated organs and glands; and the genito-urinary system. In some organs the epithelium consists of a simple layer of cuboidal or columnar cells — as in the alimentary canal, gall bladder, and airways — while in others there are multiple layers of cells arising from a germinal epithelium, with many dead cells in the outermost layers — as in the skin. In man, the skin forms an impervious and protective layer. Important absorptive and secretory functions are carried out by epithelia lining the body organs. After eating, products of digestion are absorbed from the stomach and small intestine, and glands secrete fluid and enzymes to help the digestive processes. In internal organs the cells are continually sloughed off and replaced by new cells. The life time of an epithelial cell in the gut is just a few days; for cells lining the airways it is about 40 days. To perform their secretory and absorptive functions, epithelial cells are equipped with a variety of ion channels, ion pumps, ion exchangers, and solute carriers, which enable transport of substances across the cell membrane. These are distributed asymmetrically in the cells: some are present exclusively in the face of the cell exposed to the lumen (cavity) of the organ or tube, while others are found on the opposite face. This arrangement of the transporters allows movement from one side of the epithelium to the other. Some epithelia are specialized for absorptive functions, moving substances from the lumen into the body fluids; others only secrete substances by transport into the lumen, and still others can have both functions.

Alan W. Cuthbert

epithelium A tissue in vertebrates consisting of closely packed cells in a sheet with little intercellular material. It covers the outer surfaces of the body and walls of the internal cavities (coeloms) and is often underlain by a basement membrane. It also forms glands and parts of sense organs. Its functions are protective, absorptive, secretory, and sensory. The types of cell vary, giving rise to squamous epithelium; ciliated epithelium; cuboidal epithelium, with cube-shaped cells; and columnar epithelium, with rectangular cells (see also stratified epithelium). Epithelium is derived from ectoderm and endoderm. Compare endothelium; mesothelium.