EPIDERMIS: FORMATION, CHARACTERISTICS, LAYERS, FUNCTIONS - ANATOMY AND PHYSIOLOGY - 2025

The epidermis is the most superficial layer of the skin and has functions mainly of protection against the presence of mechanical, chemical agents or ultraviolet light. The thickness of this layer in humans depends on the area studied, ranging from 0.1 mm in the most delicate areas to 1.5 mm in thick regions.

Structurally, it is composed of four layers or strata: corneal, granular, spiny and basal. In this last region we find cells in constant division that form keratinocytes - cells that dominate the composition of the epidermis - that will be part of the rest of the layers.

Source: Mikael Häggström, based on work by Wbensmith

Regarding its embryological origin, the epidermis comes from the superficial ectoderm and by the fourth month of gestation the four layers of the structure can already be differentiated.

characteristics

The skin is the organ that occupies the largest surface area - with an area greater than 2 m ² and weighing approximately 4 kg - so it fulfills a huge variety of functions, mainly protection.

This organ has a structure made up of two main layers: the dermis and the epidermis.

It is characterized by the presence of keratin. This protein is synthesized by a significant number of epidermal cells called keratinocytes, which are associated with thermoregulation and other forms of protection. They are the most abundant cells in the epidermis.

Other cells that are part of the epidermis, but in less quantity than keratinocytes, are melanocytes. These are responsible for the production of melanin, the molecule in charge of giving color to the skin and protecting it.

Blood or lymphatic vessels are not observed in the epidermis, since nutrition occurs in the next layer, the dermis, which is irrigated by these components.

Layers

In the epidermis we can recognize four main layers or strata. These are the horny layer, the granular layer, the spiny layer, and the basal layer. Below we will describe the most relevant characteristics of each cell stratum:

Horny layer

The outermost layer of the epidermis is the cornea. It is made up of multiple sheets of dead cells called keratinocytes. These cells are producers of a fibrous protein called keratin.

The cells that compose them are characterized by being large, flat and polyhedral in shape. They are stacked in vertical layers close to 25 layers thick, although more than 100 layers can be found on the palms of the hands and feet.

A lipid compound is responsible for uniting the cells of the horny layer in a highly compressed way, in the same way that bricks are joined with cement in a construction.

The replacement rate of these structures is quite high, as they are continually being lost and replaced.

The immediate function of this layer is protection against friction and other physical disturbances. Thanks to its waterproof properties, water loss is avoided.

When the layer is exposed to continuous disturbances - such as rubbing, for example - it tends to thicken and "callus" forms.

Grainy layer

Immediately after the corneal layer we find the granulosa, formed by sheets of keratinocytes that gradually degenerate until they die and are replaced by other cells, suffering from constant desquamation.

It is called "granulosa", since a set of dark granules composed of keratohyalin can easily be seen under the microscope.

This compound of the granules is made up of two proteins:

- Profilaggrin, which is the precursor of filaggrin, a protein that participates in the cornification of the skin

- Involucrin, related to keratinization.

There are also lamellar granules that, unlike the previous granules, can only be visualized using electron microscopy.

Inside these granules we find a large amount of polysaccharides, glycoproteins and lipids that help to unite the cells of the stratum corneum. That is, they will serve as a kind of molecular cement.

In these granules we also find enzymes with degradation functions, responsible for the eventual destruction of the cell nucleus and organelles.

Thorny cloak

The third layer of epidermis is also made up of keratinocytes. However, the shape of these cells is no longer flat, but acquires irregular shapes with numerous sides, reminiscent of different geometric shapes.

In this layer are melanocytes and other cells related to the immune response, called Langerhans cells.

Melanocytes are dendritic and pigment-producing cells. Dendrites extend into the cells of this stratum, serving as pigment conductors.

Langerhans cells are also dendritic cells. They are derived from the bone marrow and constitute about 5% of the cells of the epidermis. These cells are identical to macrophages seen in other tissues. Therefore, these cells function as typical immune barriers of the skin.

The structure of the stratum spinosum largely determines the mechanical properties of the skin, as it is resistant to mechanical damage and at the same time is quite flexible.

Basal layer

The last layer is formed by a thin layer of keratinocytes, whose shapes resemble a cube or a cylinder. It is quite active from the metabolic point of view and also from the cell division point of view. At this point the boundaries between the epidermis and the dermis are established.

The cells of the basal layer are mostly undifferentiated and are in a continuous process of proliferation.

In this layer the cells are generated that will replace those that die in the most superficial regions. That is, they are produced in this stratum and then have the ability to migrate wherever they are required. The average migration time from the basal layer is about two weeks. If the skin is injured, this process increases its speed.

For this reason, the regenerative capacity of the skin depends to a large extent on the state of the basal layer. If the layer is affected, skin grafts will be necessary.

Some body regions have an additional epidermis layer. The palms of the hands and feet are usually a little thicker thanks to the presence of another superficial layer called the lucid layer.

Training

Of the three embryonic layers, the skin has a dual formation. While the dermis develops from the mesenchyme, the epidermis develops from the superficial ectoderm.

In the early stages of development, the embryo is covered by a single layer of ectodermal cells. At six weeks gestation the epithelium undergoes division and a layer of cells called periderm appears.

Cell development continues until a third layer is formed in the intermediate zone. In the first three months, the epidermis is invaded by cells in the neural crest, which will be responsible for the synthesis of melanin.

As the fourth month of pregnancy approaches, the epidermis will already have its definitive organization in four well-defined layers.

Features

Protection

The first function of the first layer of the skin is intuitive: protection and prevention of fluid loss. This is responsible for forming a barrier against different types of possible disturbances, both physical and chemical. In addition to protecting against different classes of pathogens that could enter the body.

Photoprotection

A particular case of protection is photoprotection. The epidermis works as a barrier against ultraviolet radiation thanks to the presence of melanin, a pigment responsible for absorbing harmful radiation from the sun.

In animals, this pigment is a derivative of the aromatic amino acid tyrosine and is widely distributed in lineages.

Melanin production occurs in the basal layer of the epidermis. The molecule achieves its protection objective by mediating heat dissipation in a process called ultrafast internal conversion.

This conversion from a harmful energy to a harmless one is crucial for the protection of genetic material. This protection contributes to maintaining the integrity of DNA, since continuous exposure to radiation can cause damage to the molecule, being associated with the development of cancer.

The color of human skin is, presumably, an adaptive trait associated with the amount of sunlight they receive in the environment where they develop.

Dark skin is related to protection against intense solar radiation and fair skin to areas where capturing the little sunlight they receive is essential for the synthesis of vitamin D (see below).

Thermoregulation

Regulating temperature is a very important and laborious phenomenon faced by endothermic organisms. The skin - and therefore the epidermis - is the organ involved in this regulatory process.

Together with the dermis, this structure is capable of controlling temperature through the mechanisms of sweating (with its evaporation the body loses heat and thus reduces the temperature) and control of blood flow.

Perception

The skin is an organ rich in receptors of all kinds, which is why it intervenes in the phenomenon of perception and mediates communication between the body and its environment. These sensations include touch, pressure, temperature, and pain. In addition, it allows you to respond to these sensations.

For example, Merkel cells are rare components located in the deepest layer of the epidermis and are associated with tactile mechanoreception.

Exchange of substances

The skin is involved in the absorption and excretion of different substances such as mineral salts, urea, uric acid, lactic acid, and other waste substances. It is also responsible for mediating the transit of gases such as oxygen and carbon dioxide.

The role of the skin in respiration depends on the organism studied. In small organisms like amphibians, the skin is thin and actively participates in gas exchange, to the point that some species lack lungs. In mammals, there are specialized structures that are responsible for gas exchange.

Synthesis of vitamin D

Vitamin D is an essential steroid substance made up of four rings of carbon atoms, with quite marked structural similarities to the cholesterol molecule.

The synthesis of this vitamin occurs in the skin and for the reaction to occur it is necessary the presence of ultraviolet light from the sun. Then it travels to other organs (kidney and liver) to continue processing and pass the active form.

The synthesis of vitamin D is not restricted to the skin region, it can also come from foods that are included in the diet, such as fish oil or dairy foods enriched with this vitamin.

Participates in the metabolic pathway of calcium, phosphorus and in the mineralization process of bones. Its function is not restricted to the development and maintenance of the bone system, it also participates in the immune, endocrine and cardiovascular systems.

Vitamin D deficiency has been associated with rickets and osteomalacia; the first pathology is common in early ages, while the second is associated with adults. It can also cause osteoporosis, different types of cancer, multiple sclerosis or cardiovascular diseases, among other pathologies.

Wound self-repair

The skin is not only the largest organ of the human being, but it is also the first that establishes a direct contact with the environment, so it is constantly exposed to physical and chemical entities that could injure it and cause injuries.

These wounds can be repaired in a matter of days (depending on the magnitude of it) thanks to the fact that the skin has a very accelerated cell division and tissue renewal system.

Nonbiological function in humans

In the medical field, evaluating the condition of the skin provides very valuable information, since it is a true reflection of the patient's state of health and can be helpful in the identification of certain pathologies.

In addition, human skin also plays a crucial role in aesthetics and in providing each individual with a feeling of identity.

References

1. Audesirk, T., Audesirk, G., & Byers, BE (2003). Biology: Life on Earth. Pearson education.
2. Callen, JP, Jorizzo, JL, Bolognia, JL, Piette, W., & Zone, JJ (2009). Dermatological Signs of Internal Disease E-Book: Expert Consult-Online and Print. Elsevier Health Sciences.
3. Freeman, S. (2016). Biological science. Pearson.
4. Hickman, CP, Roberts, LS, Larson, A., Ober, WC, & Garrison, C. (2007). Integrated Principles of Zoology. McGraw-Hill.
5. Hill, RW, Wyse, GA, Anderson, M., & Anderson, M. (2004). Animal physiology. Sinauer Associates.
6. Junqueira, LC, Carneiro, J., & Kelley, RO (2003). Basic histology: text & atlas. McGraw-Hill.
7. Lesmes, JD (2007). Clinical-functional evaluation of human body movement. Panamerican Medical Ed.
8. Marks, JG, & Miller, JJ (2017). Lookingbill and Marks' Principles of Dermatology E-Book. Elsevier Health Sciences.
9. Randall, D., Burggren, W., French, K., & Eckert, R. (2002). Eckert animal physiology. Macmillan.
10. Rastogi SC (2007). Essentials of Animal Physiology. New Age International Publishers.
11. Ross, MH, & Pawlina, W. (2006). Histology. Lippincott Williams & Wilkins.