Epithelium is a membranous tissue that lines most of the internal and external surfaces throughout the body.

The functions of epithelial cells include protecting the internal environment from the external environment, the formation of glands, secretion, sensation detection, selective permeability, and the movement of materials in, out, or around the body.

Epithelium is categorized as a primary body tissue along with connective tissue, muscle tissue, and nervous tissue. These tissues comprise the four basic tissue-types in all animals.

The following factors determine the classification of an epithelial cell:

SHAPE

• Squamous: Squamous cells are flat and irregular in shape. These cells form the alveoli of the respiratory membrane and the lining of blood vessels. They can also be found in the filtration tubules of the kidneys. Squamous cells, with their irregular shape and centrally located nuclei, look like sunny-side-up eggs.
• Cuboidal: These cells, as indicated by their name, have a shape similar to a cube. The smallest duct glands (a gland that secretes externally through a duct) and many kidney tubules are composed of cuboidal epithelium. The edges of these cells aren’t always apparent, but their nuclei are quite pronounced.
• Columnar: Columnar cells are taller than they are wide. The small intestine and respiratory tracts are lined with columnar epithelium. Goblet cells (unicellular glands) are present throughout columnar epithelium to secret mucus.
• Transitional: Organs that can stretch, such as the bladder, utilize this special type of epithelium because its cells can slide over one another. The appearance of transitional epithelium depends on whether the organ is contracted or distended. When the organ is contracted the sliding cells give the impression that there are several layers of cells when, in fact, there might be as few as one.

STRATIFICATION

• Simple: Simple epithelium is made of a single layer of cells.
• Stratified: Stratified epithelium contains more than one layer of cells. These cells will typically resist large amounts of stress.
• Pseudostratified: Pseudostratified epithelium is made of a single layer of cells, but may appear as though it is stratified because of the location of the nuclei within the cells.

The epithelium lining the bronchi in mammals, for instance, is composed of three cell types whose nuclei are located in three distinct locations. Without proper observation, pseudostratified is difficult to classify.