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| Epithelial Tissues |
Epithelial tissues are widespread throughout the body. They cover all body surfaces inside and out. They are also the major tissues of glands. 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 the tissue is anchored to connective tissue by a thin, nonliving layer called the basement membrane. As a rule, epithelial tissues lack blood vessels. However, they are nourished by substances that diffuse from underling connective tissues, which are well supplied with blood vessels.
Although the cells of some tissues have limited abilities to reproduce, those of epithelium reproduce readily Injuries to epithelium are likely to heal rapidly as new cells replace lost or damaged ones. Skin cells and the cells that line the stomach and intestines, for example, are continually being damaged and replaced.
Epithelial cells are tightly packed, and there is little intercellular material between them. Often they are attached to one another by desmosomes. Consequently, these cells can provide effective protective barriers, such as the outer layer of the skin and the lining of the mouth. Other epithelial functions include secretion, absorption, excretion, and sensors' reception.
Epithelial tissues are classified according to the specialized shapes, arrangements, and functions of their cells. For example, epithelial tissues that are composed of single layers of cells are called simple, those with many layers of cells are said to be stratified, those with thin, flattened cells are called squamous, those with cubelike cells are called cuboidal. and those with elongated cells are called columnar.
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