Understand the skin Part 1: The epidermis

The vast majority of us are constantly looking for new products to discover to improve the appearance of our skin. Cream, exfoliant, cleanser, mask, foundation, etc., there are more product classes than letters of the alphabet, and all without exception claim to have an impact of near or far on the skin. So we thought that it would be interesting and especially relevant to understand what is made of the skin and how this organ works! In 2 parts ... we present you: the skin.

You surely know by name, the three main parts of the skin:
  1. The most superficial layer, the epidermis.
  2. The inner part which is the thickest, the dermis.
  3. The deeper layer, the hypodermis.
* Note that the hypodermis consists mainly of fatty tissue, and will not be the subject of a specific article on this journal.
To begin, we present you the epidermis!
From a cosmetic point of view, the epidermis is very important because it is this layer that gives the skin its texture, its hydration and contributes to its color. Although it is very thin (we speak about 1mm according to the parts of the body: O) it constitutes the barrier of protection between the outside world and our organism.
It is in its deepest layer (the basal layer) that keratinocytes (cells that make up the majority of the epidermis) are born. While progressing in the epidermis, they will mature and develop specific characteristics. Their course will end in the most superficial layer of the skin: the stratum corneum. The latter is composed mainly of dead and flattened cells that are regularly eliminated by a process called desquamation. It is also this layer which shelters the pores of the sweat glands and the openings of the sebaceous glands.
The horny layer is fascinating. One could honestly have written a complete article about it. To fully understand this layer, imagine a brick wall. The bricks represent the keratinocytes, and the mortar, lipids and proteins that hold the whole together.
What you need to know is that in this layer, the cells are mature and have completed the keratinization, also called the cell cycle. A normal cycle lasts between 26 and 42 days. If it is disturbed during the process, it can result in various pathologies, including dry skin. The cell cycle lengthens with age and the cells of the upper layers are older and their functions are affected. This is unfortunately one of the reasons why the skin may look more dry and flatter as you get older.
The main function of the stratum corneum is to act as a shield of protection. One of his ways of protecting is to prevent the loss of transepidermal water (TEWL) and to regulate the water contained in the skin. The two main components that enable it to play this role are lipids and the NMF (natural moisturizing factor). This gives the stratum corneum its humectant properties (ability to retain water). It is composed of components that are very soluble in water and that allow it to absorb a large amount of water even if the climate is very dry. Lipids (cholesterol, fatty acids, squalene, triglycerides, etc.) also play a role in the prevention of TEWL as well as in the prevention of the entry of dangerous bacteria. Age, genetics, seasons and diet have an impact on the amount of lipids contained in the skin. To miss it predisposes the individual to have a dry skin.
In summary, the epidermis is one of the parts of the skin that is super important in the defense against external aggressions and plays a major role in the hydration of the skin. A well-hydrated skin seems more radiant, young and healthy, hence the interest of understanding the functioning of the epidermis and especially to understand the role of its various constituents.
Baumann, L. Cosmetic Dermatology, 2nd edition, 2009.
Downind DT, Stauss JS, Pochi PE. In the chemical composition of human skin surface lipids. J Invest Dermatol. 1969: 53: 322.
Egelrud T. Desquamations. In: Loden M, Maibaich H., eds. Dry skin and moisturizers. 1st. Ed. Boca Raton, FL: CRC Press; 2000-110.
Elias PM. Epidermal lipids, barrier function, and desquamations. J Invest Dermatol.1983: 80: 44s.
Fritsch P, Schuler G, Hintner H (eds): Immunodeficiency and Skin. European Society for Dermatological Research, Immunodeficiency and Skin, Clinically Oriented Symposium, Innsbruck 1988. Curr Probl Dermatol. Basel, Karger, 1989, flight 18, pp 10-21

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