Cells clearly unlike epidermal keratinocytes begin to appear in the embryonic epidermis early in the 6th week of gestation, but the cells are so undifferentiated that it is presently impossible to identify them. Melanocytes begin to express differentiation-specific antigens (HMB-45) during the 7th week of gestation. When melanocytes are first recognizable, they are present in large numbers (~1000/mm2) and evenly distributed throughout the epidermis, suggesting that they had arrived and had organized themselves well before they expressed a marker that allowed their unambiguous identification.
HMB-45
HMB-45 is a cytoplasmic protein expressed on fetal epidermal melanocytes (not on the migrating cells) and malignant melanoma cells, but not on normal adult melanocytes.