EPITHELIAL CELLS: TYPES, FUNCTIONS - ANATOMY AND PHYSIOLOGY - 2024

The epithelial cells are a cell type responsible for coating the surfaces of the body, both external and internal. One of the most notable characteristics present in the organs of animals is the delimitation by these cellular barriers. This boundary is made up of epithelial cells.

These cell units form cohesive layers to cover the different tissues. The epithelium includes the epidermis (skin) and is also found on the surfaces of the components of the digestive, respiratory, reproductive, urinary systems and other body cavities. It also includes the secretory cells of the glands.

Epithelial cells function as a protective barrier and help protect the body from the entry of pathogenic organisms that could cause infections.

They do not only have isolation and restriction functions; They are complex structures that also have functions related to absorption and secretion.

General characteristics

The cells of the epithelium have the following characteristics:

- Epithelia can be derived from the three germ layers of an embryo: the ectoderm, the mesoderm and the endoderm.

- With the exception of the teeth, the anterior surface of the iris and the articular cartilage, the epithelium covers all the surfaces of the body, such as the skin, canals, liver, among others.

- Nutrients are not acquired through vessels or the lymphatic system. They are obtained by a simple process of particle diffusion.

- There is a constant renewal of the epithelial cells by processes of cell division.

- Epithelial cells are connected to each other by different types of junctions, mainly tight junctions, demosomes and cleft junctions. The most relevant properties of the epithelium occur thanks to these unions.

Types

Epithelia are classified according to the number of layers that compose them: simple, stratified and pseudostratified.

Simple epithelium

The simple ones are made up of only one layer of cells. Depending on the cell shape, it is subdivided into: simple squamous, simple cubic and simple cylindrical.

This classification is given by the shape of the cells that lay the tissue. Squamous cells are similar to flat plaques. Those of the cuboidal type have a similar width and height, similar to cubes. Columns have a height greater than the width.

Some examples are the epithelia that line the blood vessels, the pericardium, the pleura, among others.

In these cells two ends can be differentiated: one apical, which faces the open space or the interior of the organ; and the basal surface, located in the junctional tissue.

The epithelia usually rest on a lamina called the basement membrane (or basal lamina). This differentiation is mediated by a reorganization of the microtubule system.

Stratified epithelium

Stratified epithelia have more than one layer. The same secondary classification of simple epithelia is applied according to cell shape: stratified squamous, stratified cubic, and stratified columnar epithelium.

Stratified squamous epithelium can be keratinized at different levels. The esophagus and vagina are examples of this type of moderately keratinized epithelium, while the skin is considered "highly keratinized."

Pseudostratified epithelium

Finally, the pseudostratified epithelium is composed of columnar and basal cells located on the basement membrane. The trachea and urinary tract belong to this group.

Features

Protection

The main function of the epithelium is to provide protection and form a barrier between the environment and the interior of the body. The skin represents a protective organ.

The cell wall formed by these cells allows the escape of pathogens and unfavorable environmental conditions that could negatively affect organisms, such as desiccation.

Absorption

In mammals there are epithelial cells covering the surfaces of the intestines. The apical end is located in the intestinal cavity. Food particles pass through this area and must be absorbed by the epithelium in order to reach the blood vessels.

These cells often have microvilli. These projections from cell membranes increase the absorption surface. This area is called “brush border”, since the microvilli resemble the bristles of a brush.

Transportation of materials

In epithelia, molecules can travel from one side to the other. They can do this through two main pathways: transcellular or paracellular.

The transcellular pathway is through cells, crossing two cell membranes. In contrast, the paracellular pathway involves the passage of molecules between cells with the participation of tight junctions.

Secretion

There are epithelial cells in the glands that perform secretory functions, such as the tissue that makes up the salivary glands or the liver.

The glandular epithelium is classified into endocrine and exocrine. The exocrine secretes its products to the outside, while the endocrine does it to the blood. Therefore, these cells are closely related to the blood capillaries.

Gas exchange

Gas exchange occurs within the lung, specifically in the pulmonary alveoli, in the alveolar space.

The pseudostratified epithelium, with the presence of cilia of the respiratory system, mediates this process. In addition, this fabric prevents the entry of dust particles or pathogens that could enter the inspirations. These unwanted particles stick to the mucus film.

Immune system

Different surfaces, such as the mucosa of the intestine, the respiratory tract and the urogenital tract, are key points for the entry of potentially pathogenic microorganisms. The cells of the epithelium form a physical barrier that prevents the entry of these organisms.

However, the protective function goes beyond the barrier. Epithelial cells function as molecular sensors against the entry of pathogens and microbial infections.

When some damage or injury occurs in the epithelial tissue, an inflammatory chemical response is initiated. The deterioration of the tissue results in a series of molecules that attract defense cells in the host.

The antimicrobial activity of the tissue also includes the ability of certain glands to produce bactericidal substances. A clear example is the production of lysozyme in different secretions (saliva, tears, among others).

Recent research has shown that epithelial cells in humans can express a certain protein that increases permeability. This component is antimicrobial and helps in the elimination of Gram negative bacteria. The protein is able to bind to the typical lipopolysaccharides present on the cell surface of these bacteria.

References

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