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Epithelial Tissue

Occurring in sheets of tightly packed cells, epithelial covers the outside of the body and lines organs and cavities within the body. The cells of an epithelial tissue, or epithelium, are closely joined, with little material between them. In many epithelia, the cells are riveted together by tight junctions. This tight packing enables the epithelium to function as a barrier against mechanical injury, microbes, and fluid loss. Some epithelia, called glandular epithela, absorb or secrete chemical solutions. For example, the glandular epithelia that line the lumen of the digestive and respiratory tracts form a mucous membrane; they secrete mucus that lubricates the surface and keeps it moist.

Two criteria for classifying epithelia are the number of cell layers and the shape of the cells on the exposed surface. A simple epithelium has a single layer of cells, whereas a stratified epithelium has multiple tiers of cells. A "psuedostratified" epithelium is single-layered but appears stratified because the cells vary in length. The shape of the cells at the exposed surface may be cuboidal, columnar, or squamous.

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