Stratified Squamous Epithelium


Stratified Squamous Epithelium

A stratified squamous epithelium consists of more than one (typically many) layers, or strata, of epithelial cells (see figure). The basal layer of epithelial cells are small and cuboidal-to-columnar; the cells gradually become larger and more squamous as the cells migrate from the basal layer to the apical layer.

        Stratified squamous epithelia are specialized to withstand the mechanical stresses of abrasion. The apical layers of epithelial are designed to give way to abrasive forces, protecting the deeper tissues from the mechanical stress. As the apical layers of cells give way, they are continuously replaced by the deeper layers of epithelial cells, all of which are derived from the highly mitotic cuboidal cells of the basal layer.

Two major subtypes of stratified squamous epithelial membranes are found in the animal body:

  • Nonkeratinizing Stratified Squamous Epithelia: the epithelial cells lack large quantities of the protein keratin. A nonkeratinizing stratified squamous epithelium serves as a wet lining capable of withstanding relatively moderate abrasive stresses.
     
  • Keratinizing Stratified Squamous Epithelia: As the epithelial cells migrate from the basal layer to apical layer, they accumulate large quantities of the protein keratin. A keratinizing stratified squamous epithelium serves as a dry covering capable of withstanding relatively severe abrasive stresses.

Nonkeratinizing Stratified Squamous Epithelium

 A nonkeratinizing stratified squamous epithelium serves as a wet lining capable of withstanding relatively moderate degrees of abrasive stresses. The apical surface of this epithelium is bathed by mucus derived from glands found deep to the epithelium. The mucus moistens and lubricates the surface.

        The apical layers of nonkeratinizing epithelial cells are sloughed off the surface of the epithelium through the application of mild abrasive forces, thus protecting the deeper tissues from the mechanical stress. As the apical layers of cells give way, they are continuously replaced by the deeper layers of epithelial cells, all of which are derived from the highly mitotic cuboidal cells of the basal layer.

        As the cells are pushed up from the basal layer, the cells enlarge due to the accumulation of cytoplasm, they flatten, and they differentiate, a process that contributes to their vulnerability to abrasive forces. These cell are NOT keratinocytes; they do not normally accumulate keratin. As a result, the apical surface of the tissue must be constantly bathed by mucus and they tissue is effective only in protecting the underlying tissue against relatively mild or moderate degress of abrasion.

        A nonkeratinizing stratified squamous epithelium is found at three prominent sites in the animal body:

  • lining the esophagus,
  • lining the sides and floor of the oral cavity, and
  • lining the vagina.

All of these sites share the characteristic that they are exposed to mild-moderate degress of abrasion.

Keratinizing Stratified Squamous Epithelium

 A keratinizing stratified squamous epithelium is specialized to cover the body; it is a dry covering capable of withstanding relatively high degrees of abrasion, it prevents rapid dessication of the body and it is waterproof.

          The surface of this epithelium is characteristically dry. While thin skin containing hair follicles is kept supple with oils from sebaceous glands, the surface is free of mucus. Mucus is not needed because this epithelium relies on other tissue features to prevent the dessication of deeper tissues.

         The vast majority of the epithelial cells in this epitheial lining are keratinocytes. Keratinocytes produce and fill with keratin, a mechanically tough, fibrillar, protein of the intermediate filament family of cytoskeletal proteins. The keratin provides the mechanical strength that enables the tissue to withstand greater degrees of abrasive force than withstood by the nonkeratinizing variety of this tissue. The cells also produce and secrete a glycolipid that adsorbs to the extracellular surface of the plasma membrane to fill interstitial spaces, making the tissue waterproof.

        The apical layers of this keratinizing stratified squamous epithelial membrane are dead, cornified keratinocytes completely filled with keratin. They are sloughed off the surface when acted upon by abrasive forces. As the apical layers of keratinocytes give way, they are continuously replaced by the deeper layers of keratinocytes, all of which are derived from the highly mitotic cells of the basal layer.

        As the keratinocytes are pushed up from the basal layer, the cells differentiate, a process that primarily involves the accumulation of keratin, which provides mechanical strengt to the tissue, but which eventually interferes with the metabolic processes of the cells, leading to cell death. This differentiation process is revealed histologically as several tissue strata: stratum basale, stratum spinosum, startum granulosum, stratum lucidum and stratum corneum. Each stratum is marked by a milestone in the differentiation process.

Proceed to your microscopic study of stratified squamous epithelia. 

 

 

 


Copyright © 2009 Stephen Gallik, Ph. D.