The mechanism involved in the development of the pigmentary pattern in piebald (s/s) mouse embryos was investigated through the use of a combination grafting method. Skin from a potentially white spotted area of piebald embryos ranging in age from 11 days to birth was combined with 9-day neural tubes. Following grafting of the tissues to chick host embryos, melanoblasts migrated from the neural tube into the developing skin, and the appearance of pigment in the hair follicles and interfollicular epidermis of the grafted skin was determined. Eleven-day s/s skin produced pigmented hair in only 16% of the cases, and no melanocytes were found in the interfollicular epidermis. By 13 days 79% of the grafts formed pigmented hair while the epidermis remained pigment-free. Combination grafts recovered from implanting embryonic skin 16 days and older produced pigment regularly both in the hair follicles and the interfollicular epidermis. Control grafts of skin only were devoid of pigment in all ages tested.
These results indicate that an inhibitor of melanoblast differentiation or survival is present in piebald skin at 11 days of embryonic age. Conditions favorable for melanoblast differentiation appear in s/s skin by 13 days and, by 1 6 days, melanoblasts undergo differentiation in both the hair follicle and the epidermis. A mechanism of action of piebald during development is proposed.