They are made up of sub-units called epithelial cells which are collectively referred to as epithelium. The cells are closely packed and arranged in layers always resting on a basement membrane. Epithelial tissues are one of the simplest animal tissue and it demonstrate how individual cells can be build up into tissue of varying complexity. Epithelial tissue functions to protect underlining structures so as to prevent them from abrasion or pressure and from infection. They have a very rapid rate of cell division so that lost cells are quickly replaced.
According to the shape and size of epithelial cells ; they can be grouped into the following:
-Cuboidal epithelium
-Columnal epithelium
-Ciliated epithelium
-Glandular epithelium
-Striated or stratified epithelium
-Transitional epithelium