Essentials of Anatomy and Physiology

STRATIFIED SQUAMOUS EPITHELIUM

Stratified squamous epitheliumconsists of many
layers of mostly flat cells, although lower cells are
rounded. Mitosis takes place in the lowest layer to
continually produce new cells to replace those worn
off the surface (see Fig. 4–2). This type of epithelium
makes up the epidermis of the skin, where it is called
“keratinizing” because the protein keratin is produced,
and the surface cells are dead. Stratified squamous
epithelium of the non-keratinizing type lines the oral
cavity, the esophagus, and, in women, the vagina. In
these locations the surface cells are living and make up

the mucous membranes of these organs. In all of its

body locations, this tissue is a barrier to microorgan-

isms because the cells of which it is made are very

close together. The more specialized functions of the

epidermis will be covered in the next chapter.

TRANSITIONAL EPITHELIUM

Transitional epitheliumis a type of stratified epithe-

lium in which the surface cells change shape from

round to squamous. The urinary bladder is lined with

transitional epithelium. When the bladder is empty,

the surface cells are rounded (see Fig. 4–2). As the

Tissues and Membranes 71

Free surface

Stratified squamous

Connective tissues

Transitional

Simple squamous

Example: Lung (approximately 430X)

Alveolar sacs

Example: Esophagus

(approximately 430X)

B

Free surface

Example: Urinary bladder (approximately 430X)

A C

Figure 4–2. Epithelial tissues. (A) Simple squamous. (B) Stratified squamous.

(C) Transitional.

QUESTION:Which two of these tissues seem to be most related in structure?