Dry Skin
The concept of dry skin has also never been clearly defined. The term “dry skin” conceals
several complementary or opposite points of view . It remains completely different from the
way it is approached. People connect this notion to the effects observed and to their sensorial
dimension. Therefore, for them it is first of all a feeling of drying along with loss of skin
suppleness and elasticity, characterized by a rough appearance often associated with an important desquamation, and leading to a certain discomfort they intend to correct by using
moisturizing products. For the biologist, the xerosis would be first the consequence of a change of the coherence
and functionality of corneocytes, the water deficiency of the superficial layers of the stratum
corneum, when it exists, only resulting from it.
As a matter of fact, the physiopathogeny of most xerosis is still badly known, and it
remains difficult to distinguish the causes from the consequences of these skin abnormalities (5).
As it has been said before, in normal condition, the corneal layer is made up of a regular
assembly of corneocytes, forming a structure of modulated thickness with unique physical
qualities.
Each corneocyte contains dampening substances called NMFs (natural moisturizing
factors), resulting from the enzymatic degradation of the fillagrines, which fix a certain
quantity of inter-corneocytar water and therefore exert a decreasing osmotic pressure as they
migrate to the surface .
Any decrease in the enzymatic function therefore plays an important part on the NMF
content and consequently on the osmotic pressure and on the opening of corneosomes,
consequently easing a disorganized desquamation as it is observed with xerosis (5).
This dysfunction actually depends on a qualitative and quantitative change of enzymes
and/or on an inadequate change of the pH of the corneum (6). The inter-corneocytar cohesion
also depends on a complex mixture of lipids that constitute the lamellar structure interposed
between the corneocytes (made up of fatty acids, sterols, and ceramides coming from the
keratinosomes).
Whereas most of the research focused on the study of the change of the function of the horny layer and of its constitution and led to the theory of moisture balance (7–12), few works have been undertaken to better understand the components of the epidermal cells that are involved in skin drying. Such works will enable better understanding of the mechanisms that lead to xerosis.