The epithelium, also known as epithelial tissue, Is a compound of cells that do not have intercellular content that separates them, and is found in all of the membranes that cover both the internal and external surfaces of the body.
Together with other tissues, this set of cells plays a very important role in embryonic development and in the conformation of different organs. Below we will see what the epithelium is, what functions it performs and what are some of its main features.
What is the epithelium
The term that historically precedes “epithelium” is that of “epithelium”, which was invented by Dutch botanist and anatomist Frederik Ruysch by dissecting a corpse. With the term “epithelial,” Ruysch referred to the tissue that covered different areas of the body that 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 made up of adjacent cells (Side by side, without intracellular elements separating them), forming a kind of lamellae.
These cells, also called “epithelial cells”, they are attached to a thin membrane. From the latter, they form on the surface of cavities and structures that pass through the body, as well as on different 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 main passages and cavities of the body, the digestive tract, the respiratory tract, the urogenital tract, the pulmonary cavities, the heart cavity and the abdomen. cavity).
Regarding the layer of cells that covers the cavities, the epithelium is called the “mesothelium”. On the other hand, with regard to the internal surfaces of blood vessels, the epithelium is called “endothelium”. However, not all internal surfaces are covered with epithelium; for example, joint cavities, tendon sheaths and mucous sacs are not (Genesser, 1986).
What all types of epithelium have in common is that, although avascular, they grow on connective tissue rich in vessels. Epithelia are separated from this connective tissue by an extracellular layer that supports them, called the basement membrane.
Origin and associated tissues
Epithelium arises from embryonic development in conjunction with another type of tissue known as mesenchyme. The two tissues function to form almost every organ in the body, from hair to teeth to the digestive tract.
In addition, epithelial cells they contribute significantly to the development of the embryo from the early stages, they specifically play an important role in the development of the glands during this process. The activity exerted jointly by the epithelium and the mesenchyme is called the epithelial-mesenchymal interaction.
Although epithelial tissue does not contain blood vessels (it is avascular), it does contain nerves, so plays an important role in receiving nerve signals, As well as absorbing, protecting and secreting different substances depending on the precise place where it is located. The specific functions of the epithelium are directly linked to its morphology.
In other words, depending on the specific structure of an epithelium, this will perform the functions of secretion, protection, secretion or transport. We can then see the functions of the epithelium depending on where they are located:
1. On free surfaces
On free surfaces, the epithelium has the overall purpose of protecting the body. This protection is against mechanical damage, before the entry of microorganisms or before the loss of water by evaporation. Also, and for the sensitive purposes it contains, it is responsible for regulating the sense of touch.
2. On internal surfaces
In most internal surfaces, the function of the epithelium is to absorb, secrete and transport; even if in some others it only serves as a barrier.
Types of epithelial cells
The epithelium is classified in several ways, depending on its distribution, shape and functions. That is, they can distinguish different types of epithelium depending on the cells that make it up, depending on the specific place they are in or depending on the type of layer they form.
For example, according to Genesser (1986), we can divide the epithelium into different types the amount of extracellular layers it contains, and according to its morphology:
- Simple epithelium, made up of a single layer of cells.
- Stratified epithelium, if there are two or more layers.
In turn, simple and stratified eithelia can be subdivided according to their shape into cubic or columnar epithelium, as we will see below:
1. Simple flat epithelium
Composed of flat, flattened cells, this epithelium it is found for example in the kidneys and in large chambers like those of the heart, As well as in all blood vessels.
2. Simple cubic epithelium
Composed of almost square cells with a spherical nucleus and is located in the thyroid gland, in the renal tubules and in the ovaries.
3. Simple columnar epithelium,
With columnar cells and oval nuclei, located at the base of cells.
4. Stratified cubic epithelium
It is rare but is found in the layers of the conductors of the sweat glands.
5. Stratified columnar epithelium
With deep cell layers and lies in the excretory conductors of the large glands.
6. Transition epithelium
It is so called because before it was considered to be between the laminate and the cylindrical, it is in the urinary tract and bladderSo, 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. Pan-American Editorial: Barcelona.