Development and homeostasis of 'resident' myeloid cells: the case of the Langerhans cell

Langerhans cells (LCs) are myeloid cells of the epidermis, featured in immunology textbooks as bone marrow-derived antigen-presenting dendritic cells (DCs). A new picture of LC origin, homeostasis and function has emerged, however, after genetic labelling and conditional cell ablation models in mice. LC precursors are recruited into the fetal epidermis, where they differentiate and proliferate in situ. In adults, LCs proliferate at steady state, and during inflammation, in response to signals from neighbouring cells. Here we review the experimental evidence that support either extra-embryonic yolk sac (YS) macrophages or hematopoietic stem cells (HSCs) as the origin of LCs. Beyond LC biology, we propose that YS and HSCs can contribute to the development of distinct subsets of macrophages and DCs.