Mechanisms of photoaging and chronological skin aging.

Human skin, like all other organs, undergoes chronological aging. In addition, unlike other organs, skin is in direct contact with the environment and therefore undergoes aging as a consequence of environmental damage. The primary environmental factor that causes human skin aging is UV irradiation from the sun. This sun-induced skin aging (photoaging), like chronological aging, is a cumulative process. However, unlike chronological aging, which depends on the passage of time per se, photoaging depends primarily on the degree of sun exposure and skin pigment. Individuals who have outdoor lifestyles, live in sunny climates, and are lightly pigmented will experience the greatest degree of photoaging. During the last decade, substantial progress has been made in understanding cellular and molecular mechanisms that bring about chronological aging and photoaging. This emerging information reveals that chronological aging and photoaging share fundamental molecular pathways. These new insights regarding convergence of the molecular basis of chronological aging and photoaging provide exciting new opportunities for the development of new anti-aging therapies. This article reviews our current understanding and presents new data about the molecular pathways that mediate skin damage by UV irradiation and by the passage of time.