PRINCIPLES OF SKIN HYDRATION


If all dermatologists insist on the importance of skin hydration as an anti-aging factor, it is because "skin hydration is fundamental to the physiological function of the skin barrier against external aggressions" (JP Marty).

Moreover, hydration and respect of the cutaneous barrier are essential to limit the appearance of wrinkles (1, 2).

Simply put, it is the dermis that hydrates the epidermis by diffusion of water. Drinking is therefore the first essential gesture for a good hydration of the skin. The epidermis ensures skin hydration by retaining water both in depth, thanks to the horny layer, and on the surface, thanks to the hydrolipidic film.

Skin hydration is therefore only possible thanks to the specific lipids, which guarantee anti-dehydration and the "internal" water. In the event of a lipid deficiency, the skin can become dry, rough and the protective barrier is no longer watertight.

FOR A BETTER UNDERSTANDING, LET'S GO INTO A LITTLE MORE DETAIL


The hydration is very irregular depending on the layers of the skin. The dermis contains about 80% water and as it moves upwards towards the epidermis, the water content decreases. It is about 60% in the basal layer and only about 10% on the surface, in the horny layer (3, 4).

If it is the dermis that hydrates the epidermis, by diffusion, and not the other way around, it is the horny layer and the hydrolipidic film covering it that make the skin impermeable and limit evaporation (PIE: insensible water loss), guaranteeing good skin hydration.

The stratum corneum is often compared to a wall, the 20 layers of corneocytes* arranged in scales being the bricks bound together by intercellular lipid cement.

The 3 key elements involved in skin hydration are perfectly explained by JP Marty (1) :

  • The intercellular lipid cement that limits insensible water loss or PIE,
  • Natural Moisturizing Factors (NMF) that ensure the capture of surface water
  • The hydrolipidic film that maintains the physiological pH and protects against daily aggressions
INTERCELLULAR LIPID CEMENT, "THE PROTECTIVE BARRIER"

It binds the corneocytes together, ensuring the impermeability of the stratum corneum and limiting insensible water loss. It is synthesized by keratinocytes* during their maturation from the basal layer to the stratum corneum and by the secretion of sebaceous glands. The role of lipid cement is therefore protective, hydro-retentive or anti-dehydration, with particular importance for ceramides, polar lipids mostly present. It is the cement of the protective wall that is the skin.

* corneocytes : dead and flattened cells that are eliminated by desquamation, it is the superficial renewal of the skin. These are the cells resulting from the maturation of keratinocytes, which change their name at the horny layer.

* Keratinocytes : the majority of the cells that make up the epidermis (80%) are born in the first sub-layer, the basal layer, and are constantly renewed according to a cycle of 21 to 28 days. They differentiate progressively and during their maturation, release NMF (natural moisturizing factors) and epidermal lipids.

NMF OR NATURAL MOISTURIZING FACTORS

Water-soluble and highly hygroscopic substances, they are also synthesized by keratinocytes during their differentiation/maturation. They will retain water at the level of the horny layer and within the hydrolipidic film where they will be discharged. Lactic acid, amino acids, and mainly pyrrolidone carboxylic acid (PCA) are present.

THE HYDRO-LIPIDIC FILM COVERING THE STRATUM CORNEUM

It is a water-in-oil emulsion, therefore mostly greasy. Sweat is the aqueous phase, sebum the fatty phase. This film, with a slightly acid pH (<6) adapted to protect the skin from the penetration of bacteria and fungi, is the ultimate barrier against dehydration. The NMFs in the film retain water by hygroscopy/capture. It is however strongly and directly subjected to external aggressions (wind, cold, UVs) and to cleaning that is too alkaline or too strong. It is then necessary to help it to reconstitute itself in order to preserve to the skin its natural protection, its hydration and its healthy aspect.

These 3 elements guarantee quality skin hydration.

Thus, moisturizing the skin means avoiding dehydration thanks to epidermal lipids and maintaining the quality of its natural moisturizing components (NMF).

This will effectively ensure the skin's barrier role.

(1) La peau : une interface par « excellence ». Pr. JP.Marty, Dr C.Laverdet. Dermoscopie n°6 ? Mai 2007.
(2) Actualités cosmétiques dans le vieillissement cutané ? A. Cohen-Letessier-Annales de dermatologie et vénéréologie, vol 136 n° S6-Oct 2009.
(3) Thèse en pharmacie Université de Nancy 1- 2008. A. Georgel
(4) Leaflet Peau et galénique Sept 2013. Fondation Dermatite atopique
(5) Histologie de la peau et de ses annexes ? Cedef ? Mai 2011

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