A cut was made on the middorsal skin of mice of various ages of strain C57BL/0J using fine iridectomy scissors. Specimens from the wounded skins were fixed at various days after wounding and were subjected to the dopa reaction and to the combined dopa-premelanin reaction. When the dorsal skins of 1.5-day-old mice were wounded, the melanocyte population positive to the dopa reaction as well as the melanoblast-melanocyte population positive to the combined dopa-premelanin reaction increased dramatically in the epidermis adjacent to a skin wound. Pigment-producing melanocytes in mitosis were frequently found in the vicinity of a wound immediately after wounding. When the dorsal skins of 4.5-day-old mice were wounded, the increase in the melanocyte and melanoblast-melanocyte populations was smaller than that of 1.5-day-old mice. The increase in number of pigment-producing melanocytes in mitosis was reduced and delayed as compared to 1.5-day-old mice. When the dorsal skins of 8.5-, 20.5-, and 60.5-day-old mice were wounded, the increase in the melanocyte and melanoblast-melanocyte populations was much smaller than the newborn mice. Moreover, pigment-producing melanocytes in mitosis were never found. These results indicate that the proliferative response of mouse epidermal melanocytes to skin wounding becomes delayed and diminished with development.

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