Morphology and function of resident macrophages in hematopoietic tissues.

Resident macrophages in hematopoietic tissues, which are derived from precursor cells in the bone marrow, are components of the stroma. These macrophages are clearly differentiated from reticular cells or endothelial cells by the macrophage-specific antigen. Erythroblasts adhere to these macrophages via VLA-4-VCAM-1 and proliferate and differentiate in the presence of erythropoietin. Each macrophage that adhered to erythroblasts extends cytoplasmic processes so as to enfold erythroblasts, resulting in the formation of an “erythroblastic island”. This type of interaction between hematopoietic cells and macrophages via adhesion molecules may be important in the regulation of hematopoiesis.