Keratinocytes: what they are, functions and phases of development of these cells

The skin is the largest organ in the human body. With a surface area of ​​about two square meters and a total weight of up to five kilograms, this tissue conglomerate is the biological barrier most important primary component of complex living beings, along with mucosa, saliva, tears, sweat, and certain behavioral mechanisms (such as cough).

The skin is an inclement environment for pathogenic agents, as it is dry, has a slightly acidic pH, has properties antiseptics and, in addition, there are other microorganisms that already colonize this layer without causing us any harm (staphylococci, micrococci and Acinetobacter). All this makes the task very difficult for bacteria and parasites that want to take advantage of us, since they find themselves with a practically insurmountable physiological and living barrier.

Therefore, it is not surprising to learn that the vast majority of skin infections begin with a wound: when a crack opens Within this wall, both commensal and pathogenic bacteria take advantage of the new unprotected environments that open up under the barrier. injury. After all, the innermost layers of our skin are irrigated and contain thousands of living cells: for a parasite, this equates to unlimited nutrients.

Beyond functionalities, pathogenic agents, physicochemical properties and immune mechanisms, To understand the nature of the skin we must go to its most external and well-known part: the epidermis. In it, there is a very striking cell group, which dominates and defines the tissue. Let's see what keratinocytes consist of.

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What are keratinocytes?

As we said, Keratinocytes are the most abundant cell type in the human epidermis.. In our species, they account for 95% of the cell bodies of this layer, accompanied in a much smaller proportion by melanocytes, Langerhans cells and Merkel cells.

As its own name indicates, the keratinocytes They are in charge of synthesizing keratin and, in turn, give the pertinent properties to each of the four layers of the epidermis.: basal layer, stratum spinosum, stratum granulosum and horny layer. As a curiosity, it should be noted that the passage of a cell from the basal layer to the horny zone is about 15 days long, extremely rapid period if we look at the rate of tissue turnover in other parts of the body.

Keratinocyte characteristics

First of all, it should be noted that This cell type has ectodermal origin, that is, it comes from the distal outermost layer of the fetus and the first to develop.. They are a cell type that releases very little matrix and therefore the membranes of neighboring cells are tightly bound. This makes all the evolutionary sense in the world: the less space left between the bricks of a wall, the more difficult it is for cracks to appear.

In addition to physical proximity, it should be noted that there are a series of junctions called desmosomes between keratinocytes. This "bridge" is mediated by cadherins to a series of intermediate filaments (keratin), which allow the union between cells, thus giving the epidermis a very resistant cohesion and integrity in the time.

The classic keratinocyte is made up of 72-80% water, cytoplasm, organelles, nucleus and the expression of various types of keratin., depending on your location.

It is necessary to note, at this point, that keratinocytes do not have a specific shape throughout their lives (which in the human being is one month), since they pass through different epidermal layers and, therefore, must adapt to different functionalities. In order to show you what these cell bodies are like at each stage and stratum, we must show you, even in broad strokes, what the keratinization process consists of. Go for it.

Summary keratinization

The terminal differentiation of keratinocytes from the basal layer to the corneum occurs under a process known as "keratinization".. We will see its particularities in a superficial way layer by layer.

1. basal layer

It is the first layer of the epidermis, specifically, the only one in which melanocytes appear, more or less at a rate of one for every 23 keratinocytes. This layer is conceived as a true tissue factory, since it is made up of only one row of keratinocytes that gradually divide, in order to populate the following layers.

These keratinocytes are attached to the basal lamina (which separates the dermis from the epidermis) by hemidesmosome-type junctions, so a cell pole is clearly differentiated from the other. We do not want to go into histological particulars, but it is enough to know that the adult epithelial stem cells are found in this layer, which give rise to keratinocytes. To give you an idea, there is usually one stem cell for every 3,500 keratinocytes in this layer.

2. spinous stratum

It originates from the mitotic division of the cells of the basal layer, so it is located immediately above it.. In this section, the keratinocytes assume a polyhedral shape about 15 micrometers in diameter, larger and more turgid than those present in the basal layer. The name "spiny" comes from the desmosome-like junctions and tonofibrils that communicate cells with each other.

Notably, as they progress through the strata, keratinocytes express distinct keratin-like cytoplasmic proteins. If in the basal stratum K5 and K14 dominated, here we find K1 and K10.

3. granular layer

in this layer an important event takes place: gene expression (the synthesis of substances encoded by nuclear DNA) of keratinocytes changes. In the stratum granulosum, these cell types synthesize keratohyalin granules, basophilic compounds irregular in nature that occur in the cytoplasm of keratinocytes in this layer (hence their name). The typical keratin types of this phase are K2 and K11.

4. stratum corneum

In the stratum corneum, keratinocytes differentiate and degenerate, giving rise to corneocytes.. They do not have a nucleus or cytoplasmic organelles: they only have a thick membrane and multiple lipids, necessary for the structure. They are approximately 50 micrometers in diameter (they are larger than the rest) and are organized in columns of 10 to 30 units to form the stratum corneum itself.

Notably, in addition to losing the nucleus and organelles, corneocytes retain no more than 15% water by weight, compared with 70% for keratinocytes in the basement membrane. This gives the outermost layer of the epidermis its necessary dryness, very important for many microorganisms to be unable to colonize it.

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Its relationship with the immune system

As you can see, one of the most striking functions of keratinocytes is to “die” to become insurmountable biological barriers, but this is not the only one of its essential tasks.

It should be noted that the invasion of pathogens or contact with allergens from the epidermis remove the more "immune" side of the keratinocytes. These produce a plethora of cytokines, proteins of a proinflammatory nature, such as interleukins (IL)-1, -6, -7, -8, -10, -12, -15, -18, and -20. These cytokines attract immune bodies such as monocytes or T lymphocytes to the site, which begin to act and/or divide in order to destroy the pathogen.

Pathologies known to all rest on this physiological basis, such as contact dermatitis. When the immune system recognizes a harmless allergen as harmful, lymphocytes travel to the skin surface and produce local responses, such as itching, swelling and rash characteristic. Although it may not seem like it, the immune system is fighting an unfounded pathogen.

Summary

As you have seen, keratinocytes have both a direct and an indirect protective function. Not only are they structural "plugs" in their final phase due to their large size and low water content, but they are also capable of secrete substances that alert the immune system that something is wrong and promote the appearance of local reactions, for better and for worse evil.

Can you imagine what would have become of the human being without a series of mechanisms as effective as those exposed here? In a three-dimensional environment in which even the sun attacks us, understanding life without keratinocytes and the epidermis would be an impossible task.

Bibliographic references:

• Barker, J. N., Griffiths, C. AND. M., Nickoloff, B. J., Mitra, R. S., & Dixit, V. m. (1991). Keratinocytes as initiators of inflammation. The Lancet, 337(8735), 211-214.
• Benhadou, F., Mintoff, D., & Del Marmol, V. (2019). Psoriasis: keratinocytes or immune cells–which is the trigger?. Dermatology, 235(2), 91-100.
• Cervellati, F., Benedusi, M., Manarini, F., Woodby, B., Russo, M., Valacchi, G., & Pietrogrande, M. c. (2020). Proinflammatory properties and oxidative effects of atmospheric particle components in human keratinocytes. Chemosphere, 240, 124746.
• Eckert, R. L., Crish, J. F., & Robinson, N. TO. (1997). The epidermal keratinocyte as a model for the study of gene regulation and cell differentiation. Physiological reviews, 77(2), 397-424.
• Furue, M., Furue, K., Tsuji, G., & Nakahara, T. (2020). Interleukin-17A and keratinocytes in psoriasis. International journal of molecular sciences, 21(4), 1275.
• Oviedo Zegarra, C. TO. (2019). Analysis of the use of keratin as a biomaterial.