Homeostasis belongs to the main principles of biological process as it deals with maintaining our organism functions and constants (temperature, hydrous balance). In the skin, homeostasis occurs in the epidermis and corresponds to the keratinocytes’ proliferation and differentiation equilibrium: the continuous renewal of the basal keratinocytes layers is offset by the loss of corneocytes in the stratum corneum and the intermediate differentiation effects on cohesion of cells in epidermis. As a result, homeostasis helps to preserve skin barrier function, either for external aggressions such as penetration of antigens or for protection of the inner equilibrium by regulating the transepidermal water loss (TEWL).
Many molecules (growth factors, hormones and trace elements such as zinc or copper) are involved in regulating homeostasis. Disorders can happen biologically (aging, physiological dysfunction such as psoriasis) or can be induced by contact with irritant products. These conditions lead to a modifi cation of the multiplication speed of the basal keratinocytes and can infl uence the differentiation process to corneocytes and affect the stratum corneum thickness. These effects come from the infl ammation induction and the associate immunological disorders. This state is very difficult to treat, because it is an auto-induced phenomenon that follows an amplifying cycle. For cosmetic applications, inflammation is a very important concern because it can be initialized by many causes, including can provoke its initialization (aging, UV or pollution).