Epithelial tissue (also epithelium) occurs throughout the body. It is of two basic types: (1) that forming the epidermis; (2) that forming the surfaces of the mucous and serous membranes. It covers the body and also lines body cavities and hollow organs. It is also one of the prominent tissue types in glands. It can have various functions, including diffusion, protection, absorption, secretion, excretion, filtration, and sensory reception. In comparison with other types of cells, epithelium has a greater capacity to regenerate itself when damaged.
Epithelial cells fit tightly together with little intercellular space. Epithelial tissue, because it forms coverings and linings, has one free surface not in contact with other cells. On the opposite surface, the epithelium is attached to underlying connective tissue by a non-cellular basement membrane, composed of carbohydrates and proteins.
In shape, epithelial cells may be squamous (such tissue is also known as "pavement epithelium: because the cells of which it is composed are wide and thin like paving stones), columnar (cells that are long and thin like columns), or cuboidal (cells that are shaped approximately like cubes). There can be a single layer (simple epithelium) or multiple layers (stratified epithelium).
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