HOW EPITHELIAL TISSUES ARE CLASSIFIED:

Epithelia are classified on the basis of cell shape and arrangement. The cell shapes are columnar (column shaped cells), cuboidal (cube shaped cells), and squamous (scale-like cells).

Generally, the arrangements are simple and stratified. Simple epithelia have one layer of cells attached to a basement membrane. Stratified epithelia have two or more layers of cells attached to a basement membrane.

So, if you see a tissue that consists of a free surface, a single layer of "cube" shaped cells, and a basement membrane, then you have a simple cuboidal epithelium.

 

If you see a tissue with a free surface, three sheets of cuboidal cells, and a basement membrane, then you have a stratified cuboidal epithelium.

 

It is important to note that stratified epithelia are named according to the cells forming the free surface. So, if you see a tissue section that, going from deep to superficial, consists of a basement membrane, three layers of cuboidal cells, and ten layers of squamous cells, then you would have "stratified squamous epithelial tissue".

 

There are exceptions to the shape/arrangement rule. For example, there is a "pseudostratified" epithelium (basically, pseudo means "fake"), which is simple columnar epithelium that gives the appearance of being stratified. There is also a "transitional" epithelium, which consists of rounded (or "umbrella" shaped) cells that slide over one another and allow an organ to stretch, such as in the urinary bladder. This type of tissue may appear to be simple cuboidal at one time and simple squamous at another due to its constant movement (hence, "transitional").

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