The epithelium is usually the layer of cells 'closer' to the outside world. The outermost layer of our skin is composed of dead squamous epithelial cells, as are the mucous membranes lining the inside of mouths and body cavities. Other epithelial cells line the insides of the lungs, the gastrointestinal tract and the reproductive and urinary tracts.
Functions of epithelial cells include secretion, absorption and protection. Some epithelial cells have cilia, which act to increase the surface area.
Types of epithelium
Epithelial cells are named according to their shape. If there is more than one layer of cells (such as skin), it is stratified; otherwise, it is simple. There are also pseudostratified cells, which "appear" to consist of more than one layer.