The cellular regeneration of skin epidermis and hair follicles is maintained along with the lifespan of individuals by different adult stem/progenitor cell subpopulations localized within the specialized microenvironments, niches in the basal layer of the epidermis, sebaceous gland, and hair follicle bulge region. These small subpopulations of immature cells endowed with a high self-renewal capacity and multilineage differentiation ability contribute to replenishing the different skin cell lineages, including mature and specialized keratinocytes, sebocytes or melanocytes, in homeostatic conditions or after skin injuries.
A chronic or cumulative ultraviolet (UV) light exposure, inflammation, and oxidative stress on skin stem/progenitor cells and their progenies, usually lead to their dysfunctions via senescence or apoptosis with advancing age. This age-related decline in skin regenerative cells may result in a skin homeostatic imbalance and severe cutaneous disorders. In particular, a long-term exposure of skin cells to environmental insults such as UV radiations and internal metabolic reactions typically generates highly toxic products and reactive free radicals.