This type of epithelium is classified as such because it is multiple cell layers in thickness, the cells of its most superficial layer are squamous in shape, and it lacks keratinization at its free surface.

• Take a look at the flattened cells and their nuclei (arrows) in the superficial layers of these examples of esophageal and corneal epithelia. Note the striking differences in cell shape and nuclear morphology in the surface layers compared to the deeper layers of these epithelia.
• This type of epithelium functions to provide protection and serve as a lining or covering for moist internal cavities of the body (e.g. vagina, oral cavity, and esophagus).
• Scan the epithelium lining the larynx, cornea, esophagus, and vagina, to see several examples of stratified squamous, non-keratinized epithelium. Notice how the flattened cells in the most superficial layer still have their nuclei.

Now for some clinical relevance

• When subjected to prolonged irritation, an epithelium may transform or undergo metaplasia to a different type of epithelium. This is a defense mechanism in which the metaplastic epithelium is sturdier and better able to withstand the chronic irritation.
• Examples of metaplasia include the transformation of tracheal ciliated pseudostratified columnar epithelium to stratified squamous in heavy smokers and esophageal stratified squamous epithelium to simple columnar (mucus-secreting) epithelium in individuals with chronic gastro-esophageal reflux disorder.
   

Stratified keratinized squamous epithelium.