Figure 1.

Origin and self-renewal of macrophage. Tissue macrophages have dual origins. One part develops from embryonic progenitors in the yolk sac and fetal liver and self-renew. The other part derives from hematopoietic stem cells (HSCs) in bone marrow and blood monocyte intermediates. HSCs also can self-replenish themselves. Monocyte-derived macrophages can give rise to some subsets of resident macrophages under certain conditions. Resident macrophages and monocyte-derived macrophages ultimately constitute macrophages in all tissues, such as microglia in the brain, Langerhans cells in the skin, and Kupffer cells in the liver. EMPs, erythromyeloid progenitors; HSCs, hematopoietic stem cells.