Epithelia are usually classified under the following categories:

• Simple epithelia, formed of a single layer of squamous, cuboidal or columnar cells. The endothelium lining the lumen of vessels can also be considered simple squamous epithelium.
• Stratified epithelia, formed of two or more superposed layers of cells, the superficial ones being squamous, cuboidal or columnar.
• Pseudostratified epithelia, formed of superposed layers of cells but in this case all epithelial cells are in contact with the basement membrane, hence the term pseudostratified. Examples showing the pseudostratified columnar ciliated epithelium of respiratory passages and the transitional epithelium of urinary passages are illustrated in the present chapter.

The simplified classification of epithelia given above is not exhaustive. Indeed, some epithelia are so complex structurally that they cannot be classified in any one of these three categories. This is the case for the sensory organ of Corti in the inner ear and for the epithelium lining testicular tubules, which produce spermatozoa. These and other special epithelia will be examined later in this Atlas.

In histological sections from 10 to 15 µm thick, it is not always easy to identify epithelia for the following reasons:

• The epithelia are rarely cut perpendicular to their surface. They are usually cut obliquely or tangentially and therefore the images selected to illustrate the various epithelia in the present chapter are not necessarily representative of the images usually seen in histological sections.
• The epithelial cells often vary in shape or may be intermediate between two given categories of cell types. Thus for example, low columnar cells may be difficult to discriminate from tall cuboidal cells. Therefore, as in most classifications, the categories of epithelia that will be illustrated below are not absolutely rigid.

Before histological images of various types of epithelia are presented, most of which are shown at high magnifications, a section of the epididymis is shown at six different magnifications: 3, 6, 70, 200, 300 and 800 times.

The epididymis, located next to the testis, is composed of a single highly convoluted duct in which are stored the spermatozoa, produced by the seminiferous tubules of the testis. The epididymal duct is lined with tall columnar epithelial cells approximately 100 µm or 0.1 mm in height.

Comparison of these six images should help the reader appreciate what is actually seen in a section of a coiled duct and its epithelium at various magnifications of the light microscope.