Objective To establish and evaluate a stable and efficient animal model of cutaneous hyperpigmentation

induced by ultraviolet （UV） radiation. Methods Male C57BL/6J mice， aged 6-8 weeks， were used in this study. Following dorsal hair removal， the mice were randomly assigned to either the control （sham） group （n=5） or the ultraviolet-irradiation （UV） group （n=5）. To establish the skin hyperpigmentation model， mice in the UV group were subjected to periodic UV exposure （200 s per treatment， three times per week） for a total of nine times over three weeks. The sham group was exposed only to ambient indoor light and received no experimental UV radiation. Dorsal skin tissues were obtained 24 h after the final irradiation. Histological alterations in the skin were assessed via HE staining. Melanin deposition was evaluated through gross observation and Masson-Fontana （MF） staining. Furthermore， the expression of tyrosinase （TYR） in skin tissues was detected by immunofluorescence （IF）， while the protein levels of microphthalmia-associated transcription factor （MITF） were analyzed using western blot. Results UV exposure for three weeks did not induce significant pathological changes in the structure of the mouse skin. Gross observation and MF staining demonstrated a significant increase in melanin deposition in the UV group compared to the sham group （P<0.01）. Molecular analysis revealed that the protein expression levels of TYR （P<0.001）， and MITF （P<0.05） were significantly up-regulated in the UV group. Conclusion Regular UV exposure induces robust melanin accumulation in C57BL/6J mice without destroying skin structure. This modeling approach offers a short induction period， high stability， and distinct phenotypic presentation， serving as an ideal animal model for investigating the mechanisms underlying hyperpigmentation.

Hyperpigmentation, Animal model, UV exposure