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epithelium(ĕp'əthē`lēəm), 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.
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 Cilia and flagella
Classification of epithelia is based on morphology, that is, on the shape of the cells and their arrangement (see illustration):
- I. Single-layered.
- A. Squamous (mesothelium, descending loop of Henle in the kidney)—thin, flat.
- B. Cuboidal (duct, thyroid, choroid plexus)—cubelike.
- C. Columnar (intestine), sometimes ciliated (Fallopian tube, or oviduct)—tall.
- II. Multiple-layered or stratified.
- A. Squamous (skin, esophagus, vagina)—superficial cells thin and flat, deeper cells cuboidal and columnar.
- B. Columnar (pharynx, large ducts of salivary glands), sometimes ciliated (larynx)—two or more layers of tall cells.
- III. 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.
- IV. Transitional (urinary bladder)—like stratified squamous in the fully distended bladder; in the empty bladder, superficial cells rounded, almost spherical.
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 Gland
(1) A tissue of multicellular animals. The epithelium, found on the body surface, also lines all cavities of the body as a layer of cells. It is the prevalent component of glands. The epithelium has a high capacity for regeneration. The two main types of epithelium are the stratified (laminated) epithelium and the glandular epithelium. The epithelium is underlain by a basal membrane. It contains no blood vessels but receives nourishment from the underlying connective tissue.
The epithelium performs various functions: demarcation, protection, absorption, excretion, and secretion. The structure of the cells reflects functional specialization. For example, cells specializing in absorption have a brush border of microvilli that greatly increases the area of contact with the environment. Others are equipped with cilia. The protective epithelium has the capacity to form horny scales, and the glandular epithelium is distinguished by its well-developed granular endoplasmic reticulum and Golgi apparatus.
(2) In plants, a layer of thin-walled parenchymatous excretory cells lining the inner surface of certain plant organs, for example, resin ducts in conifers.