Epithelial tissue, also known as epithelium, is sheets of tightly cells that line organs and body cavities and covers the body. Epithelial tissues are named according to the number of cell layers they have and by the shape of their cells on their free surface. A simple epithelium has a single layer, while stratified epithelium has multiple layers. The cell shape may be cuboidal, columnar, or squamous. The structure of each type of epithelium fits in function. For example, simple squamous epithelium lines the lungs’ air sacs; simple columnar epithelium lines the intestine. Human skin, pictured on the hand shown here, is a type of stratifies squamous epithelium, which is also found in esophagus lining. It has a thick layer of dead cells at the free surface. The protein keratin is deposited into these epithelial cells and makes our skin waterproof.