The classification of covering and lining epithelia is based on two or three features:

1. The number of cell layers
   • one layer: simple epithelium
   • several layers:stratified epithelium.
2. The shape of the cells
   • flat: squamous epithelium,
   • square: cuboidal epithelium,
   • rectangular: columnar epithelium,
   • if the shape changes depending of the degree of stretching of the tissue: transitional epithelium.
   • Most often, it is very hard to distinguish the cell's boundary on a light photomicrograph. To figure out what type of cells you are dealing with, look at the shape and position of the nuclei:
     • if the nuclei are flat and parallel to the free surface: squamous epithelium,
     • if the nuclei are oval and parallel to the axis of the cell and situated at its base: columnar epithelium,
     • if the nuclei are round and situated in the middle of the cell: cuboidal epithelium.
3. Specializations of their cell surfaces (cilia, microvilli, etc).

There are 2 subtypes of epithelia:

1. The simple epithelia:
   Being composed of one layer of cells only, they are very thin. They are found in areas of minimum wear and tear. Their main function is to allow passage of substances between the lumen and the surrounding tissues.

   The stratified epithelia:

   Being composed of several layers of cells, they are very thick. Their main function is to protect the tissues that they cover. The shape of the cells closest to the basement membrane is quite different from that of the cells at the top, near the lumen. Thus, the problem is: how can you further classify the stratified epithelia? By convention, stratified epithelia are further classified according to the shape of the cells at the free surface.

Each subtype of epithelia is subdivided into 4 classes which give us a total of 8 classes of epithelia that we have to learn to identify: