Dictionary:

epithelium

  (ĕp'ə-thē'lē-əm)

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Membranous tissue composed of one or more layers of cells separated by very little intercellular substance and forming the covering of most internal and external surfaces of the body and its organs.

[New Latin epithēlium : EPI– + Greek thēlē, nipple.]

One of the four primary tissues of the body, which constitutes the epidermis and the lining of respiratory, digestive, and genitourinary passages. The major characteristic of epithelium is that the cells are close together, separated by a very small amount of intercellular substance. Epithelium may be derived from any of the three primary germ layers of the very early embryo—ectoderm, entoderm, or mesoderm. With very few exceptions, epithelium is free of blood vessels.

The functions of epithelium are varied and include (1) protective function, by completely covering the external surface (including the gastrointestinal surface—and the surface of the whole pulmonary tree including the alveoli); (2) secretory function, by secreting fluids and chemical substances necessary for digestion, lubrication, protection, excretion of waste products, reproduction, and the regulation of metabolic processes of the body; (3) absorptive function, by absorbing nutritive substances and preserving water and salts of the body; (4) sensory function, by constituting important parts of sense organs, especially of smell and taste; and (5) lubricating function, by lining all the internal cavities of the body, including the peritoneum, pleura, pericardium, and the tunica vaginalis of the testis.

The forces which hold the epithelial cells together are not satisfactorily understood. The intercellular substance between the cells, also called cement substance, is undoubtedly important. The interdigitation of adjacent cell surfaces and the occurrence of intercellular bridges in certain cells may also be important in holding the cells together. Finally, in certain cells local modifications of contiguous surfaces and the intervening intercellular substances, which together form the terminal bars, may be effective in the same way.

The outstanding property of the arrangement of most of the epithelium of the body is the economy of space achieved in the face of a broad exposure of the cell surfaces. The efficiency is achieved by the presence of numerous folds, which may be gross or microscopic and temporary or permanent. A part must also be attributed to the surface specialization of the epithelial cells themselves, such as their minute, fingerlike processes. Another specialization of the surface or epithelial cell is the occurrence of motile cilia. See also Cilia and flagella.

Classification of epithelia is based on morphology, that is, on the shape of the cells and their arrangement (see illustration):

1. Single-layered.

2. A. Squamous (mesothelium, descending loop of Henle in the kidney)—thin, flat.

3. B. Cuboidal (duct, thyroid, choroid plexus)—cubelike.

4. C. Columnar (intestine), sometimes ciliated (Fallopian tube, or oviduct)—tall.

5. Multiple-layered or stratified.

6. A. Squamous (skin, esophagus, vagina)—superficial cells thin and flat, deeper cells cuboidal and columnar.

7. B. Columnar (pharynx, large ducts of salivary glands), sometimes ciliated (larynx)—two or more layers of tall cells.

8. Pseudostratified (male urethra), sometimes ciliated (respiratory passages)—all cells reach to basement membrane but some extend toward the surface only part of the way, while others reach the surface.

9. Transitional (urinary bladder)—like stratified squamous in the fully distended bladder; in the empty bladder, superficial cells rounded, almost spherical.

Cellular arrangements in epithelial tissues. (a) Squamous. (b) Cuboidal. (c) Columnar. (d) Stratified squamous. (e) Pseudostratified. (f) Transitional.

An important property of epithelium is the ability of its cells to glide over surfaces. This allows replacement of dead cells to take place in the normal state, while presenting a closed surface to the external environment; replacement is especially important in wound repair. Gliding ability is also manifested normally in the movement of cells which slide over each other in transitional epithelium, for example, when the urinary bladder is being distended or contracted. See also Gland.

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.

The structural arrangement of the various cellular components of epithelium, characterized by two basic forms: medium suprapapillary width with medium-length rete pegs, and narrow suprapapillary width with long rete pegs.

A sheet of tissue consisting of tightly bound cells lining the external surface (e.g. epidermis) or internal surface of a body cavity.

Pl. epithelia [Gr.] the cellular covering of internal and external surfaces of the body, including the lining of vessels and other small cavities. It consists of cells joined by small amounts of cementing substances. Epithelium is classified into types on the basis of the number of layers deep and the shape of the superficial cells. Standard classifications include ciliated (cilia attached), columnar (taller than wide), cuboidal (same height as width), pseudostratified (single layer but because of varying cell height appears to be more than one), simple (one cell layer), squamous (flattened, plate-like cells), stratified (more than one layer), transitional (variable number of layers apparent). Other types are listed below.

• anterior e. — the thin layer of stratified squamous cells that form the outermost layer of the cornea.
• corneal e. — see anterior epithelium (above).
• germinal e. — thickened peritoneal epithelium covering the gonad from earliest development; formerly thought to give rise to germ cells.
• glandular e. — that composed of secreting cells.
• lens e. — cuboidal epithelium covering the lens.
• pigmentary e., pigmented e. — that made of cells containing granules of pigment.
• posterior e. — the single layer of epithelial cells on the back of the cornea, between stroma and aqueous humour; the corneal endothelium.
• retinal pigment e. — see retina.
• sense e., sensory e. — see neuroepithelium (1).