Visible Red Light Enhances Physiological Anagen Entry In Vivo And Has Direct And Indirect Stimulatory Effects In Vitro

The study investigated the effects of red, green, and blue visible light on hair follicle cycling in mice and on cell proliferation in vitro. It was found that red light (630 nm) at 3 mW and 1 J/cm^2 accelerated the transition from telogen (resting phase) to anagen (growth phase) more effectively than green or blue light when applied to mice for 3 weeks. In vitro experiments showed that red light irradiation stimulated the proliferation of outer root sheath keratinocytes and dermal papilla cells in a dose-dependent manner, which was mediated through extracellular signal-regulated kinase phosphorylation. Additionally, red light-treated dermal papilla cells enhanced keratinocyte proliferation in a co-culture setup, indicating improved epithelial-mesenchymal interaction. This effect was associated with an increase in fibroblast growth factor 7 (FGF7) at both mRNA and protein levels, and the stimulative effect on keratinocytes was significantly reduced by a neutralizing antibody against FGF7. The results suggest that red light can promote the physiological transition from telogen to anagen by directly stimulating hair follicle cells and indirectly by enhancing cell communication, indicating a potential therapeutic avenue for hair growth.