In biology and medicine,<unk>  is a tissue composed of a layer of cells.<unk>  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<unk> ithelial cells.
Mucous membranes lining the inside of the mouth, the esophagus, and part of the rectum are lined by nonkeratinized stratified squamous<unk> . Other, open to outside body cavities are lined by simple squamous or columnar<unk> ithelial cells.
Other<unk> ithelial cells line the insides of the lungs, the gastrointestinal tract, the reproductive and urinary tracts, and make up the exocrine and endocrine glands.  The outer surface of the cornea is covered with fast-growing, easily-regenerated<unk> ithelial cells.
Functions of<unk> ithelial cells include<unk> cretion, absorption, protection, transcellular transport,<unk> nsation detection, and<unk> lective permeability.
Endothelium (the inner lining of blood vessels, the heart, and lymphatic vessels) is a specialized form of<unk> . Another type, Mesothelium, forms the walls of the pericardium, pleurae, and peritoneum.
In humans,<unk>  is classified as a primary body tissue, the other ones being connective tissue, muscle tissue and nervous tissue.
Epithelium
Cartilage
Junctional epithelium
Respiratory epithelium
Parenchyma
Connective tissue
Adnexa
Germ layer