Although all dermatologists stress the importance of skin hydration as an anti-ageing factor, it is because “skin hydration is fundamental to the physiological function of the skin barrier against external aggression” (J P Marty) and because hydration and respecting the skin barrier are essential for limiting the appearance of wrinkles (1, 2).
Skin barrier and hydration
Put more simply, it is the dermis which hydrates the epidermis by diffusion of water. Drinking is therefore the first essential action for good skin hydration. As for the epidermis, it provides skin hydration by retaining water both deep down, due to the stratum corneum, and on the surface, due to the hydrolipidic film.
Skin hydration is therefore only possible as a result of specific lipids, which guarantee anti-dehydration and “internal” water. In the case of lipid deficiency the skin may become dry, rough and the protective barrier is no longer watertight.
To understand this better, let’s go into more detail:
Hydration is very irregular depending on the layers making up the skin. The dermis contains about 80% water and moving up to the epidermis the water content decreases. It is around 60% in the basal layer and is no more than about 10% on the surface, in the stratum corneum (3, 4).
Although it is the dermis that hydrates the epidermis by diffusion and not the other way round, it is the stratum corneum and the hydrolipidic film covering it which makes the skin impermeable and limits evaporation (TEWL: trans-epidermal water loss), guaranteeing good skin hydration.
The stratum corneum is often compared to a
wall, the 20 layers of corneocytes* placed in scales being the bricks with the intercellular lipid cement joining them together.
The 3 key elements coming into play in skin hydration are perfectly explained by JP Marty (1):
- the intercellular lipid cement which limits the trans-epidermal water loss or TEWL
- the natural moisturising factors (NMF) which provide water collection on the surface
- the hydrolipidic film which maintains the physiological pH and protects against daily aggressions.
The intercellular lipid cement, “the protective barrier”
It interlinks the corneocytes, providing the impermeability of the stratum corneum and limiting the trans-epidermal water loss. It is synthesised by the keratinocytes* during their maturation from the basal layer to the stratum corneum and by secretion from the sebaceous glands. The role of the lipid cement is therefore protective, a water retention or anti-dehydration agent, with very special importance for ceramides, polar lipids which are present in a majority. It is the cement for the protective wall which is the skin.
* Corneocytes: dead, flattened cells which are eliminated by desquamation. This is the superficial renewal of the skin. These are cells which come from the maturation of keratinocytes, which change their name in the stratum corneum.
* Keratinocytes: cells that are the major components of the epidermis (80%). They arise in the 1st sub-layer, the basal layer, and they constantly renew themselves in a cycle of 21 to 28 days. They differentiate progressively during their maturation, release NMFs (natural moisturising factors) and epidermal lipids.
NMFs or natural moisturising factors
These are very hygroscopic, water-soluble substances. They too are synthesised by the keratinocytes during their differentiation/maturation. They will retain water in the stratum corneum and within the hydrolipidic film where they will be discharged. Lactic acid, amino-acids, and mainly pyrrolidone carboxylic acid (PCA) are found here.
The hydrolipidic film covering the stratum corneum
This is a water in oil emulsion, so mainly fat. In it the sweat is the aqueous phase and the sebum the lipid phase. This film, with a slightly acid pH (<6), is adapted to protect the skin from the penetration of bacteria and fungi. It is the ultimate barrier against dehydration. The NMFs in it retain water by hygroscopy/uptake. However, it is strongly and directly subject to external aggression (wind, cold, UV) and to cleansing that is too alkaline or too forceful. So it needs help to reconstitute itself in order to preserve the natural protection, hydration and healthy appearance of the skin.
These 3 elements guarantee good quality skin hydration. Thus moisturising the skin means preventing it from dehydration thanks to the epidermal lipids and maintaining the quality of its natural moisturising components (NMF). In this way there will be effective provision of 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