Progenitor niches and influence of differentiated progeny. (A) Microenvironment-promoting progenitors. Stromal cells labeled by Prx1-cre or Osx-cre and expressing CXCL12 (CAR cells) have been shown to regulate both lymphoid progenitor maturation and myeloid progenitor retention. Depletion of CXCL12 in Osx-cre–targeted stromal cells depletes B-lymphoid progenitor cells and induces the mobilization of myeloid progenitors from the bone marrow. Macrophages also constitute the central unit of the erythroblastic island allowing erythroblasts to mature and generate red blood cells (RBCs). (B) Regulation by HSC progeny. Macrophages promote HSC retention in the bone marrow by regulating CXCL12 production from Nes-GFP⁺ perivascular cells. Macrophage-mediated clearance of neutrophils inhibits the retention signals, which allows HSC mobilization. Regulatory T cells (T_reg) enable an immune privilege site, protecting HSCs against rejection following allogeneic transplantation. Megakaryocytes localize with HSCs and promote their quiescence. IL-10, interleukin-10.