Characterization of a composite tissue model that supports clonal growth of human melanocytes in vitro and in vivo.

To aid in the investigation of factors that control the proliferation and function of melanocytes, we have characterized a skin equivalent model that supports melanocyte growth and function in vitro and in vivo. Passenger melanocytes survive and proliferate at low numbers when keratinocytes of the epidermis are cultured in serum-containing medium using a fibroblast feeder layer. When the surface of de-epidermalized acellular dermis was seeded with these cultured cells, the keratinocytes formed a stratified epithelium in vitro containing rete ridges, and the melanocytes were preferentially located in the bottom of these rete ridges. Melanocyte cell number was much less than in normal skin, but in some areas the melanocytes were in clusters, consistent with clonal growth of the cells. When transplanted to athymic mice, the grafts formed foci of pigmentation at 3 wk that expanded and repigmented the entire graft by 8 wk. Histologic examination of these foci revealed that they corresponded to clusters of melanocytes that proliferated and migrated to eventually repopulate the entire graft. In grafts of mixed cells from light and dark skin donors, distinct foci of pigmentation were obvious at 3 wk and, instead of progressing to complete repigmentation, these foci remained stable for over 6 wk. Histologic examination confirmed that these grafts of mixed cells were entirely repopulated with melanocytes and that the grafts contained distinct zones of melanocytes that were of exclusively dark or light skin origin. This model should be valuable for studying the clonal growth of melanocytes in the context of the epidermis.