In vivo studies of the evolution of physical properties of the human skin with age.

In vivo measurements taken by valid methods, help to interpret the phenomena associated with the aging process. The authors developed some routine techniques to measure physical properties of the skin both deep in the dermis-epidermis and at the surface (stratum corneum). Measurements on the forearm of 150 people yielded the following results regarding the dermis-epidermis level: (1) skin thickness begins to decrease at 45 years of age for men and women, when women's skin becomes thinner than men's skin; (2) torsion extensibility, normalized for a given skin thickness, sharply decreases after 35 years of age; (3) skin optical properties are modified, ie the photoplethysmographic signal, measured on the forehead, greatly increase after 60 years of age; (4) at the cutaneous surface level, the main modifications are increased shedding of stratum corneum after 60 years of age in parallel with a modification of the stratum corneum cohesivity, altered skin microrelief during the aging process, and slightly decreased transepidermal water loss while the corneocyte projected size increases. These findings allow consideration of the aging process at the skin level as a phased process. The alterations of the physical properties and their kinetics suggest a various process according to the different cutaneous sections.