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Epithelium is a tissue composed of sheets of cells that are joined together in one or more layers. Epithelia cover the body surface, line body cavities and hollow organs, and form glands. Epithelial tissue forms a barrier between the body and the external environment and plays important roles in protection, filtration, absorption, excretion, and sensation. The rapid regeneration of epithelial cells is important to their protective function. Impervious barriers between cells (tight junctions) allow some epithelia (as in the gut) to tightly regulate flow of materials across them. Glands typically contain clusters of epithelial cells that either secrete their products (such as hormones ) into the bloodstream or secrete products (such as digestive enzymes ) by way of ducts onto an epithelial surface, such as the epidermis or stomach lining.

Epithelia are classified on the basis of cell shape and number of layers: Squamous cells are thin and flat, cuboidal cells are cubical to round, and columnar cells are tall and cylindrical. A simple epithelium is composed of a single layer of cells, all of which contact a nonliving basement membrane below. A stratified epithelium is composed of two or more cell layers. Each of these classes has four types of epithelium (see table below).

Simple squamous epithelium is a single layer of flat cells, simple cuboidal epithelium has a single layer of cubical cells, and simple columnar epithelium has a single layer of columnar cells. Pseudostratified columnar epithelium is a simple epithelium that looks stratified because some of its cells are shorter than others and do not reach the free surface.

Stratified epithelia are named for the shape of the cells at the surface; the deeper cells may or may not have a different shape. In stratified

Type of Epithelium Typical Locations Typical Functions
Simple Squamous lining of heart, blood vessels, and lungs filtration and secretion
Simple Cuboidal lining of kidney tubules and other ducts secretion and absorption
Simple Columnar lining of gastrointestinal tract secretion and absorption
Stratified Squamous epidermis of skin protection
Stratified Cuboidal lining of sweat gland ducts protection
Stratified Columnar lining of large ducts protection
Transitional lining of urinary bladder elastic properties
Pseudostratified Columnar lining of the upper respiratory tract secretion and movement

squamous epithelium, the surface cells are flat; in stratified cuboidal epithelium, the surface cells are cubical or round; and in stratified columnar epithelium, surface columnar cells rest on a basal layer of cuboidal cells. Transitional epithelium, a stratified type found only in the urinary tract, has cells that change shape and move across each other as an organ, such as the bladder, expands and contracts.

see also Digestive System; Gas Exchange; Skin; Tissue

Michael G. Scott


Gartner, Leslie P., and James L. Hiatt. Color Textbook of Histology. Philadelphia, PA: W. B. Saunders, Co., 1997.

Tortora, Gerard J., and Sandra R. Grabowski. Principles of Anatomy and Physiology, 9th ed. New York: John Wiley & Sons, Inc, 2000.

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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

See also glands; mucous membrane; skin.
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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.

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ep·i·the·li·um / ˌepəˈ[unvoicedth]ēlēəm/ • n. (pl. -li·a / -lēə/ ) Anat. the thin tissue forming the outer layer of a body's surface and lining the alimentary canal and other hollow structures. ∎  more specifically, the part of this derived from embryonic ectoderm and endoderm, as distinct from endothelium and mesothelium. DERIVATIVES: ep·i·the·li·al / -lēəl/ adj.

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epithelium (epi-theel-iŭm) n. the tissue that covers the external surface of the body and lines hollow structures (except blood and lymphatic vessels). Epithelium may be either simple, consisting of a single layer of cells; stratified, consisting of several cell 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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epithelium Layer of cells, closely packed to form a surface for a body tube or cavity. Epithelium covers the skin, and various internal organs and surfaces such as the intestines, nasal passages and mouth. Epithelial cells may also produce protective modifications such as hair and nails, or secrete substances such as enzymes and mucus.

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epithelium A type of animal tissue which consists of a sheet or tube of tightly packed cells with the minimum of intercellular material. Epithelia cover the exposed surfaces of the body, often line tubes and cavities, and are frequently secretory in function.

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epithelium The lining of a resin canal in a gymnosperm or a gum duct in a dicotyledon. Epithelial cells often secrete resin or gum.