Both in humans and the mouse, Langerhans cells (LC) can already be identified in the fetus. Unlike the fetal murine epidermis, which contains phenotypically immature Ia− LC until the end of gestation (1), HLA-DR+/ATPase+ dendritic cells (DC) can be identified in the human epidermis by 6 to 7 weeks of estimated gestational age (2). These cells must originate from HPC in the yolk sac or fetal liver, the primary sites of hemopoiesis during the embryonic period. Until the twelfth week of pregnancy, these cells are CD1a− and lack Birbeck granules (BG). Thereafter, there occurs a dramatic increase in LC CD1a expression, an event which coincides with the initiation of bone marrow function (2).