An epithelium is a layer of cells that are connected or arranged side by side in order to form a surface lining of a fluid or air filled tube, cavity or else a surface that is exposed to the outside, such as skin. These layers will perform various functions depending on its site and several specific structural differences can also be seen among such specialized tissue structures. One of the variations that can be seen among epitheliums is its ‘stratified’ or ‘multilayered’ nature against a simple, single layer arrangement.
-General structure of a stratified epithelium
In certain parts of the body, where the surfaces are subjected to constant strain, irritations or else friction, the single cell nature of the epithelium would not be appropriate and therefore, these cells are arranged as multiple layers on top of each other with one layer packed against the layers from above and below. Only one layer will be resting against the basement membrane through which the nutritional requirements are met. In most instances, the cells resting on the outermost edge will receive nutrition through diffusion rather than direct transport of these elements through blood vessels.
-Types of stratified epitheliums
As mentioned earlier, depending on the shape of the cells, there are three main varieties to be identified and these are, the stratified squamous epithelium, stratified columnar epithelium and stratified cuboidal epithelium.
Stratified squamous epithelium
This epithelium can be identified in the outermost layer of the skin, inner lining of the mouth, esophagus and the vagina. These structures are tolerant to frequent abrasions and therefore frictions. The epithelium has the ability to slough off the surface most layers and produce new cells from the inner most aspect. In another variation, some of the outermost layers of stratified squamous epithelium can become hardened due to keratin deposition in order to better withstand the stressors. The epithelium of the skin is one such example and the mucosal linings are usually examples for non-keratinized epitheliums.
Stratified columnar epitheliums
Stratified columnar epitheliums are found in certain parts of the body and among them the ocular conjunctiva of the eye, uterus, pharynx…etc can be considered important. The main functions of this type of epithelium would be secretary or else protective or both.
Stratified cuboidal epithelium
The cuboidal types of stratified epitheliums are found on the lining of secretary glands such as salivary, mammary and sweat glands.
Apart from the above mentioned variations, in certain parts of the body, the stratified epitheliums might show certain variations and among them, presence of cilia is a significant adaptation. These structures are useful in blocking secretions, debris as well as organisms from entering vital structures such as lungs and would push those outwards with rhythmic movements to perform a protective function in the respiratory passages.