Epithelial tissues are tissues which line the outer surface and junctions of an organ. Their name derives from epi-, an adjectival prefix for outer, and thelium (pl. thelia), which refers to a nipple, papilla, or projection. Epithelial tissues are used for cell regulation: they aid selective permeability and control the passage of materials through the body.
Epithelial tissues are identified via the number of cell layers and the shape of the epithelial cells. The number of layers in the tissue designates them as either simple epithelial tissues or stratified epithelial tissues, and the shape of their cells when viewed through a cross-section sample determines whether they are squamous, cuboidal, or columnar. They are named using the same terms, structured so the cell layer type precedes the cell shape; notably, a stratified epithelial tissue uses the shape of the cells on the topmost, or free and unattached, surface layer of the epithelial tissue.
Epithelial tissues can be identified by their dense packing of epithelial cells which aid in the regulation of selective permeability. They possess a basement membrane which attaches and anchors the tissue to its respective organ.
The category of epithelial tissues is diverse, and categorized tissues line the epidermis of several organs in the body; they may be found at a known external area, in layers of the skin, or at internal areas of passage, such as the intestines. Other tissue examples include the nephrons of the kidneys.