Epithelium, also known as epithelial tissue , is a compound of cells that lack intercellular content to separate 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 plays a very relevant role in the embryonic development and in the formation of different organs. Next we will see what the epithelium is, what functions it performs and what some of its main characteristics are.
What is 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. By the term “epithelial”, Ruysch meant the tissue that covered different areas of the body he was dissecting. 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 (one next to the other, with no intracellular elements to separate them), forming a kind of lamellae.
These cells, also called “epithelial cells”, are attached to a thin membrane . From the latter, they form to the surfaces of the cavities and structures that pass through the body, as well as various glands.
Where is it?
The epithelium is found on almost all surfaces of the body . It covers from the epidermis (the outer layer of the skin), to the membranes that cover the large pathways and cavities of the body (the digestive tract, the respiratory tract, the urogenital tract, the lung cavities, the heart 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 the blood vessels, the epithelium is known as the “endothelium. However, not all internal surfaces are covered by epithelium; for example, joint cavities, tendon sheaths and mucosal sacs are not (Genesser, 1986).
What all types of epithelium have in common is that, despite being avascular, they grow on connective tissue that is rich in vessels . The epitheliums are separated from this connective tissue by 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 known as mesenchyme. Both tissues have the function of forming almost every organ in the body, from hair to teeth and the digestive tract.
In addition, epithelial cells contribute significantly to the development of the embryo from the early stages, specifically they play an important role in the development of glands during this process. The activity carried out jointly by the epithelium and the mesenchyme is called epithelium-mesenchymal interaction.
Although the epithelial tissue does not contain blood vessels (it is avascular), what it does contain are nerves. Therefore, it plays an important role in receiving nerve signals , as well as absorbing, protecting and secreting different substances depending on the specific location. 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, protection, secretion or transport . We can then see the functions of the epithelium according to where they are located:
1. On free surfaces
On free surfaces, the epithelium has the general objective 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 sensitive endings it contains, it is in charge of regulating the sense of touch.
2. On internal surfaces
On most internal surfaces, the epithelium has the function of absorbing, secreting and transporting; although on some others it serves only as a barrier .
Epithelial cell types
Epithelium is classified in many ways, according to its distribution, shape and function. In other words, several 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 based on the number of extracellular layers it contains, and according to its 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 ethylium can be subdivided according to their shape into cubic or cylindrical epithelium, as we shall see below:
1. Simple flat epithelium
Composed of flat, flattened cells, this epithelium is found for example in the kidneys and in large chambers such as those of the heart , as well as in all blood vessels.
2. Simple cubic epithelium
Composed of almost square cells with a spherical nucleus, it is found in the thyroid gland, in the renal tubes and in the ovaries .
3. Simple cylindrical 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 the sweat gland conductors.
5. Stratified cylindrical epithelium
It has deep cellular layers and is found in excretory conductors of the large glands .
6. Transition epithelium
It is so called because it was previously considered to be between the stratified and the cylindrical, it is found in the urinary tract and in the bladder , so it is also called urothelium.
- McCord, K. (2012). Epithelium. Embryo Project Encyclopedia. Retrieved August 24. Available at http://embryo.asu.edu/handle/10776/3946.
- Genesse, F. (1986). Histology. Editorial Panamericana: Barcelona.