Murine Neonatal Melanocytes Exhibit a Heightened Proliferative Response to Ultraviolet Radiation and Migrate to the Epidermal Basal Layer

Melanocytes respond to UVR not only by producing melanin, but also by proliferating. This is essentially a protective response. We have studied the melanocyte proliferative response after a single UVR exposure to neonatal mice. At 3 days post-UVR in wild-type neonates we observed a marked melanocyte activation not seen in adults. Melanocytes migrated to the epidermal basal layer, their numbers peaking at 3–5 days after UVR then diminishing. They appeared to emanate from the hair follicle, migrating to the epidermis via the outer root sheath. In melanoma-prone mice with melanocyte-specific overexpression of HrasG12V, basal layer melanocytes were increased in size and dendricity compared to UVR-treated wild-type mice. Melanocytes in mice carrying a pRb pathway cell-cycle defect (oncogenic Cdk4R24C) did not show an enhanced response to UVR such as those carrying HrasG12V. The exquisite sensitivity to UVR-induced proliferation and migration that characterizes neonatal mouse melanocytes may partly explain the utility of this form of exposure for inducing melanoma in mice that carry oncogenic mutations.