Dry skin can be clinically defined as skin which has lost its smoothness to the touch and it’s look of smoothness. It has lost its ability to retain water.
Dry skin is skin lacking lipids (sebum), water or both. Other factors can also have an effect, such as ultraviolet rays, pollution or climate conditions.
Figure 1: Alteration of the hydrolipidic film and reduction of the protective lipids in dry skin compared to normal to mixed skin
The lack of sebum: Dry skin possesses less protective lipids on its hydrolipidic film, due to a sebaceous hyposecretion, and therefore it reacts more intensely to aggressions and these signs of ageing (wrinkles, loss of elasticity…) are more visible, due to the epidermal cell renewal.
The lack of water: A reduction in the concentration of water in the stratum corneum will occur, due to the alteration of the hydrolipidic film, to the reduction of protective intercellular lipids (ceramides) and due to the slowdown in the synthesis of Natural Moisturising Factor (NMF) ingredients (Figure 1). These ingredients are urea, uric acid, lactic acid, amino acids, sodium, potassium... situated within the cells of the stratum corneum (corneocites) and on the surface of the epidermis. Their role is to trap water within these cells.
For black and dark skin the skin barrier is higher with a stronger cell cohesion and increased number of proteins, which are linked covalently. This causes a rigidity increasing the cornified cell envelope.
The activity of this enzyme which is responsible for the peeling process, is much lower. Therefore this is slower and creates a more rough and thicker skin.
The composition of the lipid matrix involves three classes of dominant biomolecules: Ceramides (45-50%), cholesterols (25%) and fatty acids (11%).
Ceramides are extremely hydrophobic, in comparison with other membrane lipids. This hydrophobia explains their abundance in the stratum corneum. They ensure the cohesion of corneocytes to form a barrier which also minimises dehydration of the skin.
By inhibiting the action of free radicals, elastase, collagenase and by stifling the degradation of structural proteins, they contribute to repairing, improving and restructuring the texture of the skin.
The low quantity of ceramides causes a defective barrier function and an alteration in the capacity to retain water.
This weakness is mainly due to the increase in ceramidase (the enzyme which is responsible for degrading ceramides).
CONCLUSION
The association of the strong cell cohesion, reduction in the quantity of serine protease and weakened rate of ceramides makes black and dark skin extremely dry and visually scaly, and these types of skin require particular hydration.
To accomplish its mission to protect, the skin must be a resistant and impermeable barrier. This role greatly involves the epidermis, which is directly exposed to the outside environment, but the epidermis is not an impenetrable barrier. The epidermis must release water continually travelling from the dermis via capillaries to evaporate on the surface of the skin. This is known as trans-epidermal water loss (TEWL).
Conversely, the skin can allow water to enter, but it can also allow active molecules to enter in the same way. This absorbent property of the skin is used to hydrate the skin. This is what allows cosmetic actions to be effective.
Before hydrating your skin with an adapted product like MEL’OYACorrector Unifying Moisturising for the face or MEL’OYA® Lite Body Milk Unifying Moisturizing, you must exfoliate to eliminate the dead cells in the thick stratum corneum of black skin, with, for example CLEAN’OYA® EXTRA Gel Cleanser Exfoliating.
Hydration occurs thanks to:
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