Why does skin become dry?
The outermost layer of the epidermis — the stratum corneum — acts as a protective barrier in both directions! It not only protects against external aggressors, but also against water loss from the deeper layers.
The stratum corneum can be compared to a wall in which the cells — the corneocytes — act as bricks, while the cement is represented by lipids (ceramides, cholesterol, fatty acids) arranged in a liquid crystal structure. The surface also features the hydrolipidic film, a mixture of watery and lipid substances formed thanks to the secretion of sweat and sebum.
The result is a sort of wall, but rather than being a simple, static structure, it is complex and dynamic because:
- it is constantly being renewed, like all the epidermis;
- it is constantly alert to everything that happens on the outside and inside;
- it acts non-stop to organise and mobilise water.
Under “normal” conditions, the stratum corneum acts as an effective barrier, defending against physical and chemical agents as well as pathogenic bacteria; all while regulating the balance of water between the outer environment and the underlying layers of skin. A certain amount of water loss through the skin is natural; this loss is measured as the quantity of water vapour which passes from the stratum corneum outwards: TEWL, or Transepidermal Water Loss.
Low TEWL levels indicate good skin barrier integrity, while high levels are an indicator that the skin barrier has less protective power.
In the event of alterations to the proteins of the corneal cells (the bricks), the inter-corneocyte lipids (the cement), or the hydrolipidic film, the barrier function may be impaired. As a result, the water content of the epidermis is reduced and the skin becomes dry and xerotic.