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Stratified Epithelia

In some body locales, friction and other forces dictate a lining epithelium be more durable for protection.  Stratified epithelia consist of two or more layers of epithelial cells cemented together.  In the most severe environments, epithelia consist of multiple layers to provide maximum protection.

Stratified epithelia are found covering the exterior body surfaces and lining portions of the body tracts where friction phenomena occur.  A multilayered epithelium allows deeper cells to replace the more superficial cells as they are damaged or cast off from the epithelial free surface.  Although most prevalent in the skin and portions of the digestive and reproductive tract, stratified epithelia can be found in some large ducts and tubules.

Stratified Squamous Epithelium

The most prevalent of the stratified epithelia is the squamous variety.  As a "keratinized" or waterproofed component of the skin, stratified squamous epithelium provides substantial protection against  friction, microbial invasion and dessication.

Keratinized Type

Keratinized stratified squamous epithelium is identified by the multiple layers of dead cells at the surface.  Keratin accumulation within maturing cells effectively waterproofs the cells, blocking diffusion of nutrients and wastes.  The cells subsequently die.

Keratinized stratified squamous epithelium is found only in the skin!

Thick skin has many layers of these dead cells cemented together.

Thin skin has fewer layers of  living and dead cells but same structure.

Non-keratinized Mucous Type

In the oral cavity and esophagus, stratified squamous epithelia occur as a "mucous" or non-keratinized type, designed for protection and lubrication during chewing and swallowing.  This mucous variety also occurs in the rectum and lower portions of the female reproductive tract.  A key attribute of this mucous stratified squamous type is observed in the surface cells.  In contrast to the keratinized stratified squamous epithelium, flattened surface cells of a mucous type are nucleated and living.

Non-keratinized or mucous type, stratified squamous is found in:

Transitional

Transitional epithelium is a stratified type that can be confused with stratified squamous.  The name transitional comes from the ability of this epithelium to accommodate stretching in the urinary tract as fluid pressures vary.  It is found as a lining of the ureters, urethra, and bladder.  The large, ovoid surface cells are a key attribute of this epithelium type.

Transitional epithelium is only found in the urinary tract!

Transitional epithelium in the bladder!

Transitional epithelium in the urethra!

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