Epithelial Tissue

Epithelial tissues cover and line all body surfaces. They cover organs, vessels, and ducts, and line hollow organs, vessels, and ducts. As a result, all epithelial cells have a free edge that borders an open area on the outside surface or as a lining of an inside surface.

Epithelial tissues are named, first, for the shape of their cells and, second, for the amount of layering of the cells.

Cells are three-dimensional objects—with a width, height, and depth— and can have different shapes. Epithelial cells can be squamous (flat and thin), cuboidal (cube-shaped), or columnar (tall column-shaped).

A basement membrane separates epithelial tissue from other tissues.

Tissue layering is described in three ways: simple epithelial tissue has a single layer of epithelial cells; stratified epithelial tissue is composed of stacked layers of epithelial cells; and pseudostratified epithelial tissue appears to be layered, but all cells have contact with the basement membrane, so it is a false (pseudo) layering.

The following are some examples of epithelial tissue:

• Simple squamous epithelial tissue lining the alveoli (air sacs) of the lung.
• Simple cuboidal epithelial tissue that lines the tubules in the kidneys.
• Stratified squamous epithelial tissue lining the mouth and esophagus.
• Simple columnar epithelial tissue that lines the small intestine.
• Pseudostratified ciliated columnar epithelial tissue that lines much of the respiratory tract. In addition to its ciliated columnar cells that move debris in the respiratory tract, this tissue contains goblet cells that function to produce mucus.

Transitional epithelial tissue is stratified (layered), but its cell shape is difficult to describe because it is so changeable. It is designed to stretch, and it lines structures such as the urinary bladder. If stretched, the cells appear to be more squamous. If not stretched, they appear to be more cuboidal.