STRATIFIED FLAT EPITHELIUM: CHARACTERISTICS AND FUNCTION - BIOLOGY - 2024

A stratified or non-keratinized flat epithelium is a lining epithelium that is made up of a variable number of cell layers depending on the degree of protection and the location of the epithelium (number of layers greater than two). In contrast, the simple flat epithelium consists of a single layer of cells that rest on a basement membrane.

This epithelium is non-keratinized, since the nucleus and cytoplasm have not been replaced by keratin in the superficial cells; what if it occurs in the keratinized stratified epithelia present in the skin, where the superficial (dead) cells are constituted by keratin.

Source: English Wikipedia user Samir

characteristics

In flat stratified epithelia, each cell layer is superimposed on the previous ones, only the cells located deeper in the epithelial tissue being in contact with the basal lamina, while the following cell layers are not related to it.

In general, only the superficial cells of these epithelia are flat (squamous) and are named after them. The cells of the intermediate and basal layers usually vary in shape and height.

The deepest ones (those closest to the connective tissue) are usually cubic or cylindrical, varying to a polyhedral or pyramidal shape in the middle and continuously acquiring a flat shape until reaching the surface. These stratum basal cells are often larger than the superficial ones.

These characteristics are common in all epithelial tissues with slight variations depending on their location in the body. The cells that make them up are very closely united without intercellular material existing between them, which is of great importance when it comes to carrying out absorption, secretion or protection functions.

Types of stratified flat epithelia

In stratified flat epithelia, the surface, which is made up of cells that are wider and deeper than they are tall, may or may not be cornified (keratinized). In this sense, there is a clear differentiation between keratinized epithelia and those that are not cornified.

The difference between the two lies in the presence in the most superficial layer of keratin sheets, an acidophilic substance made up of dead epithelial cells but which give the tissue resistance to wear and tear and impermeability.

However, both types of flat stratified epithelia are found where body surfaces are subjected to intense mechanical stress or stress.

Location of non-keratinized flat stratified epithelium

Non-keratinized stratified epithelium is found lining interior surfaces such as the oral cavity, esophagus, vocal cords in the larynx, vagina, and anus, as well as the anterior surface of the cornea and conjunctiva, and the distal portion of the urethra of man.

The number of layers that make up the epithelium at each of these locations often varies. In the epithelium that covers the anterior surface of the cornea, there are a few very compact cell layers one above the other, clearly distinguishing basal cells, intermediate cells and superficial cells respectively.

On the other hand, in the esophagus the number of layers is higher, constituting a thicker epithelium. In addition, the keratinized flat stratified epithelium is part of the epidermis, which constitutes the upper layer of the skin and all its annexes.

Origin

The origin of these epithelial tissues is ectodermal or endodermal depending on their location.

Non-keratinized stratified flat epithelia of the mucosa of the oral cavity and the distal portion of the anal canal are of ectodermal origin, whereas the epithelium of the esophagus is of endodermal origin.

On the other hand, the keratinized stratified flat epithelium has an ectodermal origin.

Function

The main function of the non-keratinized stratified flat epithelium is to protect and function as a barrier against friction or abrasion.

In the skin (keratinized epithelium) it works as a barrier that protects the body itself from harmful physical and chemical agents and prevents it from drying out.

Protective function of vaginal stratified epithelium

In the flat stratified epithelium that lines the vagina, cells are arranged in such a way that layers or strata are formed. In total, it is made up of three layers with cells that share cytomorphological characteristics. In sexually mature women, a basal and parabasal layer, an intermediate layer and a superficial layer are recognized in the epithelium.

The vaginal epithelium plays a fundamental role in the protection of the mucosa against infections by pathogenic microorganisms. The vaginal microbiota, where some Lactobacillus dominate, is responsible for generating protection against pathogens in the female genital tract.

The recognition between surface structures of the lactobacilli called adhesins and the receptors of the epithelium, allow these lactobacilli to adhere to the vaginal epithelium, thus inhibiting the colonization of unwanted organisms.

When lactobacilli are associated with the vaginal epithelium, a kind of protective biofilm is formed, which together with the coaggregating capacity of the microbiota with potential pathogens, form the first protection against the establishment of infectious agents such as Escherichia coli, G. vaginalis and Candida albicans.

Importance of the study of the vaginal stratified flat epithelium

The stratified flat epithelium located in the vulva and vagina (vaginal epithelium), constitutes the lining of an area exposed to the external environment, but unlike the epithelium that covers the skin, in this case, as it is not keratinized, it increases its susceptibility. zone, resembling other mucous membranes.

It has been more than a century since the hormonal dependence of the vagina was discovered, since the epithelium presents rapid responses to periodic ovarian hormonal variations, thus constituting a kind of "vaginal cycle".

Changes in the concentrations of steroid hormones cause sequential modifications in the vaginal epithelium during estrous cycles, which in turn causes differences in the conditions of pH, relative humidity and composition of vaginal discharge between periods of childhood, fertile, pregnancy, and lactation and postmenopause.

Knowing the characteristics of the vaginal epithelium is of great value in endocrine functioning studies, in addition to facilitating the diagnosis of certain pathologies.

Some studies carried out in rodents have revealed that the vaginal epithelium presents cyclical changes that cause it to fluctuate between a mucified epithelium to a keratinized type. These changes are due to processes of cell proliferation, differentiation and desquamation.

References

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