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A membranous tissue composed of one or more layers of cells that form the covering of most internal and external surfaces of the body and its organs--internally, including the lining of vessels and other small cavities; externally, including the skin.
Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue, and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands.
The primary functions of epithelial tissues include:
Protection for the underlying tissues from radiation, desiccation, toxins, and physical trauma;
Regulation and exchange of chemicals between the underlying tissues and the body cavity
Epithelial layers are avascular, so they must receive nourishment via diffusion of substances from the underlying connective tissue, through the basement membrane. Epithelia can also be organized into clusters of cells that function as exocrine and endocrine glands. Exocrine and endocrine epithelial cells are highly vascular. Cells in epithelium are densely packed together like bricks in a wall, leaving very little intercellular space. The cells can form continuous sheets, which are attached to each other at many locations by tight junctions and desmosomes.
Cell junctions are especially abundant in epithelial tissues. They consist of protein complexes and provide contact between neighbouring cells, between a cell and the extracellular matrix, or they build up the paracellular barrier of epithelia and control the paracellular transport.
Types of Epithelia Tissue
Epithelial tissue that is only one cell thick is known as simple epithelium. If it is two or more cells thick, it is known as stratified epithelium. However, when taller simple epithelial cells (e.g. columnar) are viewed in cross section with several nuclei appearing at different heights, they can be confused with stratified epithelia. This kind of epithelium is therefore described as "pseudostratified" epithelium. There are three principal morphologies associated with epithelial cells:
Squamous epithelium: cells which are wider than they are tall (flat and scale-like)
Cuboidal epithelium: cells whose height and width are approximately the same (cube shaped)