The epithelium is one of the four major types of tissue found in animals, the other three being muscle tissue, connective tissue and nervous tissue.
The epithelium is also found lining blood vessels.
There are three major types of epithelium classified by cell shape into squamous epithelia (flat and wide), cuboidal epithelia (even cube shape) and columnar epithelia (tall and narrow). There is also three ways in classifying epithelia based on the number of layers that make up the tissue. Simple epithelia involve one layer of cells, stratified involves two layers of cells and pseudostratified has one layer of cells that are of different sizes and so do not appear even.