The skin is the first barrier protecting the body from external aggressions. This organ is composed of several layers of tissue. We distinguish the epidermis which is the outermost part of the skin, the dermis, a connective tissue consisting of fibroblasts and an extracellular matrix, which ensures the functions of cohesion and nutrition of the skin, and the hypoderm consists of adipocytes.
The epidermis consists of several cell layers of keratinocytes. One distinguishes, among others, the germinal layer of the epidermis, called basal layer, containing, in particular, cutaneous stem cells, the spiny layer, Stralum. spinosum, consisting of several layers of polygonal cells, the granular layer, Stralum granulosum, comprising one to three layers of flattened cells containing cytoplasmic inclusions, the grains of keratohyalin, and finally, the stratum corneum, Stralum corneum which is composed of anucleate cells and rich in keratin called comeocytes which correspond to the terminal stage of differentiation of keratinocytes.
The outermost cells of the stralum corneum are continually removed and replaced by the cells of a lower layer, according to a process called desquamation. Cellular regeneration of the stralum corneum is based on a cellular maturation process in which basal layer cells of the epidermis differentiate and migrate progressively through the different strata of the epidermis until they reach the stratum corneum under the form of comeocytes.
Skin aging, whether it results from a normal phenomenon of senescence or is accentuated by an external factor such as exposure to UV radiation, involves dysfunctions of differentiation and / or cell renewal resulting in atrophy of all the foundations of the skin.
The skin may also have a loss of elasticity and firmness, and a less luminous complexion. The skin may have other alterations in its visual appearance, especially stretch marks or redness.