General Characteristics

Epithelial tissues (ep<3i-theie-al tishcciz) are widespread throughout the body. Since epithelium covers organs, forms the inner lining of body cavities, and lines hollow organs, it always has a free surface—one that is exposed to the outside or to an open space internally. The underside of this tissue is anchored to connective tissue by a thin, nonliving layer called the basement membrane.

As a rule, epithelial tissues lack blood vessels. However, nutrients diffuse to epithelium from underlying connective tissues, which have abundant blood vessels.

Because epithelial cells readily divide, injuries heal rapidly as new cells replace lost or damaged ones. Skin cells and the cells that line the stomach and intestines are epithelial cells that are continually being damaged and replaced.

Epithelial cells are tightly packed, with little intercellular material. In many places, desmosomes attach one to another (see chapter 3, page 70). Consequently, these cells form effective protective barriers in such structures as the outer layer of the skin and the inner lining of the mouth. Other epithelial functions include secretion, absorption, and excretion.

Epithelial tissues are classified according to the shape and number of layers of cells. Epithelial tissues that are composed of single layers of cells are simple; those with two or more layers of cells are stratified; those with thin, flattened cells are squamous; those with cubelike cells are cuboidal; and those with elongated cells are columnar. In the following descriptions, note that the free surfaces of epithelial cells are modified to reflect their specialized functions.

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