Depilatory laser miniaturizes hair by inducing bystander dermal papilla cell necrosis through thermal diffusion

Depilatory laser targeting melanin has been widely applied for the treatment of hypertrichosis. Both selective photothermolysis and thermal diffusion have been proposed for its effect, but the exact mechanism of permanent hair reduction remains unclear. In this study, we explore the role of thermal diffusion in depilatory laser-induced permanent hair loss and determine whether nonpigmented cells are injured by thermal diffusion.C57BL/6 mice in anagen and telogen were treated with alexandrite laser (wavelength 755 nm, pulse duration 3 milliseconds, fluence 12 J/cm2 , spot size 12 mm), respectively. Histological analysis, terminal deoxynucleotidyl transferase dUTP nick-end labeling assay, and transmission electron microscopic imaging were employed to evaluate the injury to hair follicle (HF) cells. The proliferation status of HF cells was examined by 5-bromo-2'-deoxyuridine pulse labeling. The number of HF stem cells was quantified by fluorescence-activated cell sorting. The size of the regenerated hair was determined by measuring its length and width.We found that irradiating C57BL/6 mice in anagen with alexandrite laser led to hair miniaturization in the next anagen. In addition to thermal disruption of melanin-containing cells in the precortex region, we also detected necrosis of the adjacent nonpigmented dermal papilla cells due to thermal diffusion. Dermal papilla cells decreased by 24% after laser injury, while the number of bulge stem cells remained unchanged. When the laser was delivered to telogen HFs where no melanin was present adjacent to the dermal papilla, thermal necrosis and cell reduction were not detected in the dermal papilla and no hair miniaturization was observed.Our results suggest that depilatory laser miniaturizes hair by inducing thermal necrosis of dermal papilla cells due to secondary thermal diffusion from melanin-containing precortex cells in the anagen hair bulbs.