Epithelial cells make up the outer (i.e., skin) and inner (e.g., oral mucosa) linings of the body. They aggregate into layers of epithelium.  Epithelium may be simple (i.e., single layer), pseudostratified, or stratified into sublayers of different characteristics.
These cells are further differentiated by shape and by the presence of additional mechanisms.
Flat, thin, and irregular in shape, these are found in such places as the skin and inside the mouth and nose, and in the anal canal.
Cuboidal epithelial cells are equal in width, depth, and length, and are found in the kidney.
Epithelial cells lining the skin, mouth, nose and anal canal derive from ectoderm. Those lining the respiratory system and the gastrointestinal tract derive from endoderm. Epithelial cells in the cardiovascular and lymphatic system derive from mesoderm.