Epithelial tissue is the outer covering of the surface of the body. It can either be described as simple epithelium, meaning its layers are one cell thick, or stratified epithelium, where its layers are two or more cells thick. Each division is subdivided into three subdivisions determined by the shape of the cells: squamous, cuboidal, and columnar. Squamous cells have a flat, thin shape and are found lining the surfaces of certain organs. Cuboidal cells have a cube-like shape and are found on absorptive organs and glands. Columnar cells look like vertically lengthened cuboidal cells and are usually specialized as sensory receptors. A unique classification of simple epithelium is pseudostratified, where the location of the nuclei of simple columnar cells are irregular, so they appear to be stratified. The functions of epithelial tissue are sensation, protection of core tissue from damage, secretion of substances, absorption of nutrients and moisture, excretion of wastes, and the diffusion of gases.
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