[Natural and photo-induced aging of the skin: the three dimensional culture approach].

Human skin is a complex multifunctional organ which covers and surrounds the whole body ensuring a key function of protection against external injuries. Because of this unique situation, aging of skin is the result of both extrinsic factors-mostly sun exposure leading to photoaging- and intrinsic factors assumed to represent chronological aging. Studies of such complex phenomena on human volunteers is questionable and classical cultures of skin cells are not close enough to in vivo physiological conditions. However it is possible to address these questions by reconstructing human skin in vitro with both a living dermal equivalent defined as a fibroblast-contracted collagen gel (Bell et al., 1979) and a fully differentiated epidermis characterized by horny layers (Asselineau et al., 1985).