Epithelium
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 blood vessels and organs throughout the body.
There are three principal shapes of epithelial cells: squamous, columnar, and cuboidal. These can be arranged in a single layer of cells as simple epithelium, either squamous, columnar, cuboidal, pseudo-stratified columnar, or in layers of two or more cells deep as stratified (layered), either squamous, columnar or cuboidal. All glands are made up of epithelial cells. Functions of epithelial cells include secretion, selective absorption, protection, transcellular transport, and sensing.