Excimer Laser Penetrates Deeper into Hair Follicles and Activates More Melanocyte Lineage Cells than Excimer Light

Background: Both ultraviolet B (UVB) phototherapy with 308-nm excimer light and excimer laser devices are widely used to treat vitiligo. While the devices share the 308-nm wavelength, they have distinct characteristics. Notably, the excimer laser exhibits laser properties (monochromatic coherent light) and unique device specifications (a high frequency of 400 Hz and a remarkably high irradiance of 83 million mW/cm2). This study compared excimer light and laser irradiation, focusing on the depth of penetration into hair follicles and the effects on activation of melanocyte lineage cells, including melanocyte stem cells (McSCs) and melanoblasts. Methods: We irradiated the dorsal skin of mice with both devices at 1,000 mJ/cm2. Samples taken at 15 min and 3, 24, and 72 h after irradiation were used for immunostaining analysis. We evaluated penetration depth by using the staining pattern of cyclobutane pyrimidine dimers (CPDs), induction of apoptosis by using a terminal deoxynucleotidyl transferase-mediated deoxy-uridine triphosphate nick end-labeling (TUNEL) assay, and activation of melanocyte lineage cells by using fluorescent double immunostaining for TRP2 and β-catenin. Results: The excimer laser induced significantly more CPDs in the deeper regions of hair follicles while causing significantly faster removal of CPDs and less apoptosis in the epidermis. Moreover, the percentage of TRP2-positive cells with nuclear β-catenin in the follicles was significantly higher with the excimer laser. Conclusions: As compared with excimer light, the excimer laser penetrated more deeply into hair follicles, resulted in fewer epidermal side effects, and activated significantly more melanocyte-lineage cells.