As aging progresses, the turnover rate of the epidermis decreases. This process is associated with a decrease in the proliferative capacity of epidermal cells and a failure in differentiation control. As a result, the turnover rate of the epidermis and the stratum corneum slows down, leading to the formation of a lower-quality stratum corneum and a reduction in moisture content.
Moreover, the barrier function also diminishes. The decreased turnover delays the expulsion of melanin, becoming a factor in photoaging symptoms such as certain age spots.
During wound healing, stem cells with the abilities to move, proliferate, and differentiate migrate to the damaged area, regenerating the stratum corneum. Stimuli such as trauma or DNA damage promote pigment regeneration by causing stem cells to differentiate into melanocyte precursor cells migrating from the hair follicles.
In mouse epidermis, there have been reports suggesting that the basement membrane space formed after the differentiation of epidermal cells stimulates stem cell proliferation and promotes epidermal turnover.
Epidermal stem cells include interfollicular epidermal stem cells involved in epidermal turnover, hair follicle stem cells that supply cells to the entire hair follicle and participate in damaged epidermis repair, pigment stem cells that are melanocyte precursor cells, dermal stem cells, and mesenchymal stem cells involved in epidermal repair and regeneration.
These stem cells accumulate at the damaged site through various mediators caused by physiological epidermal layer turnover, UV stimulation, oxidative stress, and DNA damage, and are involved in tissue regeneration.
Recently, it has been reported that mesenchymal stem cells and MUSE cells enhance the proliferative capacity and oxidative stress resistance of melanocytes at the damaged site by coexisting with them.
The decline in the turnover rate of the epidermis with aging is believed to be related to the aging of epidermal stem cells. Recent research suggests that there is a depletion of stem cells with aging.
Aging is believed to lead to the thinning of the epidermal layer and the thickening of the stratum corneum due to the decreased production of collagen and integrins. Such age-related changes in epidermal stem cells, combined with the effects of ultraviolet rays, are believed to cause photoaging phenomena such as age spots and actinic keratosis.
Dr. Munenori Matsuzawa, Chief Medical Officer, Aoyama Medical Clinic.
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