Abstract: WU Zong-zhou*, YUAN Ding-fen, ZHANG Wei, CHEN Jia, BAI Mei, DENG Hui. *Department of Dermatology, Shanghai Skin Disease Hospital, Shanghai 200050, China Corresponding author: YUAN Ding-fen, Email: yuandingfen@medmail.com.cn 【Abstract】 Objective To evaluate the effect of fractional Er:YAG laser therapy on collagen and elastic fibers in photoaged skin of ultraviolet-irradiated Guinea pigs. Methods Sixty Guinea pigs were randomly divided into the non-intervention group （n = 10） and model group （n = 50） by a random number table. To establish a model of photoaging, the 50 Guinea pigs were irradiated by ultraviolet A （UVA） and ultraviolet B （UVB） on the back every other day for two months. Subsequently, the 50 mice were equally classified into five groups: photoaging group receiving no laser therapy, as well as 2-week, 4-week, 8-week and 12-week group receiving one, one, two and three sessions of successive multi-pulsed fractional Er:YAG laser therapy, respectively, on the left side （treatment side） of the back, with the right side as the control side. The interval of laser therapy was four weeks. Skin biopsies were obtained from the right and left side of the back of Guinea pigs after the laser therapy, and subjected to haematoxylin and eosin （HE） staining, Masson staining and Weigert-van Gieson staining. The content of hydroxyproline was measured by using an alkaline-hydrolysis method. Results The minimal erythema dose for UVA and UVB was 4224 mJ/cm2 and 504 mJ/cm2 respectively on Guinea pig skin. The model for photoaging was successfully established in 50 Guinea pigs after two months of irradiation with the cumulative dose of UVA and UVB being 459.36 J/cm2 and 54.81 J/cm2 respectively. HE staining revealed an obvious increase in newly-growing collagen and elastic fibers, which were arranged densely, in the treatment side compared with the control side of the back of Guinea pigs after laser therapy. The area ratio of collagen fibers stained positive for Masson′s trichrome and content of hydroxyproline were significantly higher in the treatment side than in the control side of the 12-week group （0.70 ± 0.12 vs. 0.63 ± 0.08, t = 1.18, P < 0.05; （4.73 ± 0.39） mg/g vs. （3.66 ± 0.85） mg/g, t = 3.40, P < 0.05）. Conclusion Successive multi-pulsed fractional Er:YAG laser irradiation can result in an increase in collagen fibers and renewal of elastic fibers in superficial dermis of photoaged skin of Guinea pigs. 【Key words】 Skin aging; Laser therapy, low-level; Fibrillar collagens; Elastic tissue; Radiation injuries, experimental

Key words: Radiation injuries, experimental