Epithelium: types and functions of this type of biological tissue

The epithelium, also known as epithelial tissue, is a compound of cells that lack intercellular content that separates them, and that is found in all the membranes that cover both the internal and external surfaces of the organism.

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

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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 corpse. With the term "epithelial", Ruysch designated the tissue that covered different areas in the body that 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 currently use.

Thus, in the context of modern physiology and biology, the epithelium is a type of tissue that is made up of adjacent cells (one next to another, without intracellular elements that separate them), forming a kind of sheets.

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

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Where is it located?

The epithelium is located on almost all surfaces of the body. It covers from the epidermis (the outer layer of the skin), to the membranes that line the great pathways and cavities of the body (the digestive tract, respiratory tract, urogenital tract, lung cavities, cardiac cavity and cavity abdominal).

When it comes to the layer of cells that lines 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 covered 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 vessels. The epithelia are separated from this connective tissue through an extracellular layer that supports them, called the basement membrane.

Origin and associated tissues

The epithelium originates during embryonic development in conjunction with another type of tissue that we know as mesenchyme. Both tissues have the function of forming almost every organ in the body, from hair to teeth to the digestive tract.

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

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Its functions

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

In other words, according to the specific structure of an epithelium, This will fulfill the functions of secretion, protection, secretion or transport. We can then see the functions of the epithelium according to where they are:

1. On free surfaces

On free surfaces, the epithelium has the general objective of protecting the organism. This protection is against mechanical damage, before the entry of microorganisms or before the loss of water by evaporation. Likewise, and due to the sensitive endings it contains, it regulates the sense of touch.

2. On the internal surfaces

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

Types of epithelial cells

The epithelium is classified in many ways, based on its distribution, shape, and functions. That is, several types of epithelium can be distinguished according to the cells that compose it, according to the specific place where 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 based on the amount of extracellular layers it contains, and according to its morphology:

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

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

1. Simple flat epithelium

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

2. Simple cuboidal epithelium

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

3. Simple columnar epithelium,

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

4. Stratified cuboidal epithelium

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

5. Stratified columnar epithelium

With deep cell layers and is found in excretory conductors of the great glands.

6. Transitional epithelium

It is called like this because before it was considered that it was between the stratified and the cylindrical, it is in the urinary tract and bladder, which is why it is also called the urothelium.