In biology and medicine, epithelium is a tissue composed of a layer of cells. Epithelium lines both the outside (skin) and the inside cavities and lumen of bodies. The outermost layer of our skin is composed of dead stratified squamous, keratinized epithelial cells. Mucous membranes lining the inside of the mouth, the esophagus, and part of the rectum are lined by nonkeratinized stratified squamous epithelium. Other, open to outside body cavities are lined by simple squamous or columnar epithelial cells. Other epithelial cells line the insides of the lungs, the gastrointestinal tract, the reproductive and urinary tracts, and make up the exocrine and endocrine glands.Functions of epithelial cells include secretion, absorption, protection, transcellular transport, sensation detection, and selective permeability. Endothelium (the inner lining of blood vessels) is a specialized form of epithelium.
In humans, epithelium is classified as a primary body tissue, the other ones being connective tissue, muscle tissue and nerve tissue.
