Epithelial cells: characteristics, parts and functions

Epithelia constitute one of the 4 fundamental tissues in living beings, along with the connective, the muscular and the nervous. As a whole, this type of tissue represents more than 60% of the cells present in the human body, as it covers all the free surfaces of living organisms.

A very common preconception is that the epithelium and the skin itself are the same, but nothing could be further from the truth. The epithelium lines the outer layer of the skin, but it is also present in the lining of organs and blood vessels (in this case it acquires the name of endothelium).

In addition to covering structures, epithelia (and therefore epithelial cells) have many more functions. For example, epithelial derivatives are the main secretory cells of the body, since the endocrine, exocrine and mixed glands are included within this tissue network.

To understand the characteristics and functioning of tissues, we must turn to their basic functional units: cells. Therefore, today we tell you everything you need to know about epithelial cells, including structures that allow their specificity. Do not miss it.

What is the epithelium?

Epithelial tissue is the basic or primary tissue variety made up of clusters of cells located adjacently, strongly adhered to each other, with very little extracellular matrix and a basement membrane that relates them to the connective tissue on which they are found.

The functions of the epithelium are very varied, since among them are the following: protection against desiccation / abrasion, filtration, selective absorption of substances, secretion of compounds, exchange of gases and molecules, transport of substances and sensory capacity (if it has specialized cells for this). As you can see, life as we know it would not be conceivable without epithelial tissue.

In general, we can distinguish two large types of epithelium: the simple and the stratified. The first is made up of a single layer of cells, while the second can present more than one cell alignment, arranged in several lines of nuclei. There is a third type of exceptional epithelium, the mixed one, in which the cells are presented in a more disorganized way.

What are epithelial cells like?

One of the most important characteristics to know about epithelial cells is that they are polarized. This means that they have a luminal or apical pole, whose surface is in contact with the outside of the body (in the case of the epidermis), a conduit or cavity that they cover, and a basal pole, whose surface is in contact with the basal lamina. in which the cell rests.

1. The apical part of the epithelial cells

The apical specializations of the cell give the characteristics to the epithelium itself. We will tell you about them below.

1.1. Microvilli

Microvilli are very small finger-like extensions, which help to effectively increase the surface area of ​​the cell, without this implying a compromise of its total volume. Microvilli are approximately 1 µm in diameter and, depending on the cell type, up to 2 µm in length.

As you can imagine, one of the places where more epithelial cells with microvilli is the small intestine. Thanks to these fine cytoplasmic protrusions, it is estimated that the human intestine has a useful nutrient absorption surface of about 250 square meters. Almost nothing.

1.2. Stereocilia

They are immobile and rigid microvilli that form a series of brush-shaped tufts. They measure 100 to 150 nm in diameter and are about 120 μm long at most. Its function is transport absorption of liquids and, therefore, we can observe them mainly in the epididymis (organ located at the posterior border of the testicle, where the sperm mature).

1.3. Cilia

Cilia are filiform cell expansions present, like the rest, in the luminal or apical pole of epithelial cells. Unlike the rest of those observed, these structures are about 0.25 μm in diameter and 10-15 μm in length. They tend to appear packed, like "grass", on the free surfaces of many cell tissues.

They are structures that, unlike stereocilia, can move, so they are ideal to generate currents and promote movement in fluids, with all that this entails. As a curiosity, it should be noted that in many unicellular organisms this is the only structure that allows them to move.

1.4. Flagella

Similar to cilia, but much larger (150 μm long), flagella have as their main mission displace the cell itself that presents them, instead of generating currents. They are much less numerous than cilia and, as it may have come to mind, they are found mainly in the male gametes, the sperm.

2. The basal part of the epithelial cells

Here we have much less ground to cover, since it is mostly the apical section of the cell that gives it its functionality. Even so, the basal pole is just as essential, since allows epithelial cells to rest on the basal lamina, a thin layer of extracellular matrix that separates epithelial tissue from many other specialized cell groups (such as muscle fibers or fat, for example).

Certain interesting structures can also be found at the basal pole, but we will not describe them in as much detail as in previous cases. For example, some cells present invaginations, which are more or less deep membrane folds. Others have hemidesmosomes, structures that act as "bridges", joining the epithelial layer to the basal lamina.

A continuous regenerative cycle

Epithelial cells are continuously exposed to inclement weather, whether environmental (cold, heat, humidity, radiation and pathogens) or internal (acids, blood pressure and many other things). Therefore, its regeneration rate is very fast. For this reason, their cell cycle is considered to be of very short duration.

The medical importance of epithelial cells

We leave histology and enter the world of medicine and clinical practice, as it turns out that epithelial cells can report a very interesting diagnostic use in certain cases. According to the United States National Library of Medicine, excessive presence of epithelial cells in the urine may be a sign of a kidney infection, a kidney problem, and other serious medical conditions.

The test of epithelial cells in the urine is part of a urinalysis, that is, the analysis of the urination of a patient, either as part of a routine protocol or because a pathology is suspected (especially of a renal nature). In general, a medical specialist may recommend this test for people with abdominal pain, excessive urination, back pain, or who pass foamy / bloody urine.

Tubular-like epithelial cells line the kidney, so its excessive presence in the urine can elucidate kidney damage of varying severity. The higher the proportion of cells in this biological fluid, the worse the prognosis for the patient is usually. Among some of the most common causes of epithelial cells in urine, we find the following:

• Urinary tract infections
• Infections with Candida albicans, a dimorphic fungus that is pathogenic of the human sexual organs.
• Diseases that compromise the integrity of the kidneys.
• Liver diseases.
• Certain types of cancer.

Resume

As you may have seen, talking about epithelial cells is complex to say the least, since a lining tissue of the small intestine has nothing to do with the outermost layer of the skin. Some epithelial cells present microvilli to increase the effective surface area of ​​the area in which they are found, while others associate with specific structures (glands) with secretory functions.

If we want you to keep a concept before all this terminological conglomerate, it is the following: epithelial cells are those that make up the epithelium, the type of tissue that covers all the free structures of the body. Depending on their origin and function, the structures presented at the apical and basal pole will be different between cell bodies.