Epithelium: types and functions of this type of biological tissue.

This type of tissue covers a large number of organs of the human body. Let's see what it is and what it does.

Epithelium, also known as epithelial tissue, is a composite of cells lacking intercellular content.is a composite of cells that lack the intercellular content that separates them, and is found in all membranes that cover both the inner and outer surfaces of the organism.

Together with other tissues, this group of cells has a very relevant role in embryonic development and in the conformation of different organs. Below we will see what the epithelium is, what functions it fulfills and what are some of its main characteristics.

What is the epithelium?

The term that historically precedes "epithelium" is that of "epithelial", which was coined by the Dutch botanist and anatomist Frederik Ruysch while dissecting a cadaver. With the term "epithelial", Ruysch designated the tissue that covered different areas of the body he dissected. It was not until the 19th century that the anatomist and physiologist Albrecht von Haller took up the word epithelial and gave it the name "epithelium" that we use today.

Thus, in the context of modern physiology and biology, the epithelium is a type of tissue that is composed of adjacent cells (side by side (side by side, with no intracellular elements separating them), forming a kind of sheet-like structure.

These cells, also called "epithelial cells", are attached to a thin membrane. From the latter are formed the surfaces of the cavities and structures that cross the body, as well as various glands.

Where is it found?

The epithelium is found on almost every surface of the body. It covers everything from the epidermis (the outer layer of the skin) to the membranes lining the large airways and cavities of the body (the digestive tract, the respiratory tract, the urogenital tract, the pulmonary cavities, the cardiac cavity and the abdominal cavity).

When it comes to the layer of cells lining the cavities, the epithelium is called "mesothelium". On the other hand, when it comes to the inner surfaces of Blood vessels, the epithelium is known as "endothelium". However, not all internal surfaces are lined by epithelium; for example, joint cavities, tendon sheaths and mucous sacs are not (Genesser, 1986).

What all types of epithelium have in common is that, despite being avascular, they grow on a connective tissue that is rich in blood vessels (Genesser, 1986).. The epithelia are separated from the connective tissue by an extracellular layer that supports them, called the basement membrane.

Origin and associated tissues

The epithelium originates during embryonic development together with another type of tissue known as mesenchyme. Both tissues have the function of forming almost all the organs of the body, from hair to teeth and the digestive tract.

In addition, epithelial cells contribute significantly to the development of the embryo from the earliest from the earliest stages, specifically they play an important role in the development of glands during this process. The activity carried out jointly by epithelium and mesenchyme is called epithelial-mesenchymal interaction.

Its functions

Although epithelial tissue does not contain blood vessels (it is avascular), what it does contain are nerves, thus, important role in the reception of nerve signals, as well as in absorbing, protecting and secretingand in absorbing, protecting and secreting different substances depending on the specific site in which it is located. The specific functions of the epithelium are directly related to its morphology.

In other words, depending on the specific structure of an epithelium, it will perform the functions of secretion, it will perform the functions of secretion, protection, secretion or transport.. We can then look at the functions of the epithelium according to where they are located:

1. On the free surfaces

On the free surfaces, the epithelium has the general purpose of protecting the organism. This protection is against mechanical damage, against the entry of microorganisms or against the loss of water by evaporation.. Likewise, and due to the sensory endings it contains, it is responsible for regulating the sense of touch.

2. On internal surfaces

On most internal surfaces, the epithelium has the function of absorbing, secreting and transporting; although in some others it serves only as a barrier. in some others it serves only as a barrier..

Epithelial cell types

Epithelium is classified in many ways, according to its distribution, shape and functions. That is, various types of epithelium can be distinguished according to the cells that compose it, according to the specific place in which they are located or according to the type of layer they form.

For example, according to Genesser (1986), we can divide the epithelium into different types according to the number of extracellular layers they contain, and according to their morphology:

• Simple epithelium, which is composed of a single layer of cells.
• Stratified epithelium, if there are two or more layers.

In turn, both simple and stratified epithelium can be subdivided according to their shape into cubic or cylindrical epithelium, as we will see below:

1. Simple flat epithelium

Composed of flat, flattened cells, this epithelium is found, for example, in the kidneys. found for example in the kidneys and in large cavities such as those of the heart, as well as in all blood vessels.as well as in all blood vessels.

2. Simple cubic epithelium

Composed of nearly square cells with a spherical nucleus and is found in the thyroid gland. in the thyroid gland, renal tubes and ovaries..

3. Simple columnar epithelium,

With columnar shaped cells and oval nuclei, which are located at the bases of the cells.

4. Stratified cubic epithelium.

It is rare but is found in layers of sweat gland conductors.

5. Stratified cylindrical epithelium

With deep cell layers and found in excretory in excretory ducts of large glands..

6. Transitional epithelium

So called because it was previously considered to be between stratified and columnar, it is found in the urinary tract and bladder. in the urinary tract and bladder, so it is also called urothelium.It is found in the urinary tract and bladder, so it is also called urothelium.