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 plays a very important role in the 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.
- Related article: "Types of major cells of the human body"
What is the epithelium?
The term that historically precedes it to "epithelium" is that of "epithelial", which It was coined by 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 epithelial word and gave it the name of "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 plates.
These cells, also called "epithelial cells", they are attached to a thin membrane . From this last are formed to the surfaces of the cavity and structures that cross the body, as well as different glands.
- Maybe you're interested: "20 Biology books for beginners"
Where is it located?
The epithelium is on almost all surfaces of the body . Coats from the epidermis (the outer layer of the skin), even in the membranes that line the large channels and cavities of the body (the digestive tract, the respiratory tract, the urogenital tracts, the pulmonary cavities, the cardiac cavity and the abdominal cavity ).
When it comes to the cell layer that lines the cavities, the epithelium is called the "mesothelium". On the other hand, when it comes to the internal 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 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 said 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 known as mesenchyme. Both tissues have the function of forming almost all the organs of the body, from the hair to the teeth and the digestive tract.
In addition, the epithelial cells they contribute in an important way to develop the embryo from the early stages, they specifically have an important role in the development of glands during this process. The activity carried out jointly by the epithelium and the mesenchyme is called epithelial-mesenchymal interaction.
- You may be interested: "The 3 phases of intrauterine or prenatal development: from the zygote to the fetus"
Its functions
Although the epithelial tissue does not contain blood vessels (it is avascular), what it does contain are nerves, with which, It has an important role in receiving 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 the place where they are:
1. On free surfaces
In the free surfaces, the epithelium has the general objective of protecting the organism. Said protection is against mechanical damage, before the entry of microorganisms or before the loss of water by evaporation . Likewise, and because of the sensory endings it contains, it is responsible for regulating the sense of touch.
2. On the inner surfaces
On most internal surfaces, the epithelium has the function of absorbing, secreting and transporting; even though in some others it serves only as a barrier .
Types of epithelial cells
The epithelium is classified in many ways, according to its distribution, shape and functions. That is, several types of epithelium can be distinguished according to the cells that comprise 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 from the amount 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 the simple and the stratified eìtelio can be subdivided according to their shape in cubic or cylindrical epithelium, as we will see next:
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 cubic epithelium
Composed of almost square cells with spherical nucleus and is located in the thyroid gland, in the kidney tubes and in the ovaries .
3. Simple cylindrical epithelium,
With columnar cells and oval nuclei, which are located in the bases of cells.
4. Stratified cubic epithelium
It is rare but is found in layers of sweat gland drivers.
5. Stratified cylindrical epithelium
With deep cell layers and you are in excretory conductors of large glands .
6. Transitional epithelium
It is so called because before it was considered to be between the stratified and the cylindrical, it is in the urinary tract and in the bladder , so it is also called urothelium.
Bibliographic references:
- McCord, K. (2012). Epithelium. Embryo Project Encyclopedia. Retrieved August 24. Available at // embryo.asu.edu/handle/10776/3946.
- Genesse, F. (1986). Histology. Editorial Panamericana: Barcelona.
