BIOLOGY Volume 1 - A Guide to General Biology - 2004

6. HISTOLOGY

6.3. Animal Epithelial Tissue

6.3.2. Stratified Epithelia

Stratified Epithelium

This tissue consists of multiple layers of Cells, making it thicker than Simple Epithelium and creating a relatively strong, impermeable barrier. The cells of stratified epithelium are produced through mitotic division in the germinative layer resting on the basement membrane (Fig. 6.19). The cells initially formed are cuboidal in shape, but they flatten as they are pushed toward the free surface of the tissue. Such cells are called squames. In some areas of the body, they remain non-keratinized, as in the Esophagus, where the epithelium protects underlying Tissues from mechanical damage that could result from contact with newly swallowed food. In other areas, the squames transform into a dead, cornified layer of keratin that eventually sloughs off. This is known as keratinized epithelium; it is particularly abundant on the outer surfaces of the Skin, lining the buccal cavity (Oral Cavity) and the Vagina, where it provides protection against mechanical injury.

Depending on the shape of the cells forming the stratified epithelium, it is classified as stratified squamous (found in PARTS OF THE esophagus), stratified cuboidal (in the ducts of Sweat Glands), stratified columnar (in the ducts of Mammary Glands), and transitional epithelium (in the Urinary Bladder).

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Fig. 6.19. Stratified squamous epithelium. A. Schematic diagram. B. Micrograph of stratified epithelium lining the vagina.

Transitional epithelium

This epithelium is often considered a modified stratified epithelium. It consists of 3–4 layers of cells that are uniform in size and shape, except for the flatter cells forming the free surface (Fig. 6.20). The surface cells do not slough off; instead, all the cells are capable of changing their shape as conditions change. This property is particularly important in structures subject to severe stretching, such as the urinary bladder, Ureter, and renal pelvis. The thickness of the transitional epithelium also prevents urine from leaking into surrounding tissues.

Fig. 6.20. Transitional epithelium lining the urinary bladder.