Figure 1.

Schematic illustration of the development of myeloid cells. Hematopoietic stem cells can commit to either a lymphoid or a myeloid fate through the generation of common lymphoid or myeloid progenitor cells. Common myeloid progenitors are located in the bone marrow of adults, and the yolk sac in embryos. When yolk sac-derived myeloid progenitors colonise the CNS, specific microenvironmental cues direct their differentiation into microglia. These embryonically derived microglia are able to proliferate and self-maintain until adulthood. In the bone marrow, common myeloid progenitors differentiate towards megakaryocytic/erythrocytic or granulocytic/monocytic phenotypes. In the blood, megakaryocyte/erythrocyte progenitors give rise to platelets and erythrocytes (red blood cells), while granulocyte/monocyte progenitors give rise to leukocytes, including granulocytes and monocytes. Circulating monocytes can then be recruited to sites of infection or injury in specific tissues and differentiate into macrophages or dendritic cells. During aging and certain inflammatory conditions, monocytes and other bone marrow-derived progenitors infiltrate the CNS and differentiate into microglia-like cells. It is not well understood whether these microglia-like cells persist or are a temporary addition to the existing microglial population. Listed in grey boxes are representative markers expressed by myeloid cell types. Common markers between microglia and macrophages are highlighted in yellow (MERTK, TREM2, CD68 and IBA1).