The central role of the macrophage in hemopoietic microenvironmental regulation.

Based on the macrophage-specific release using crystalline silica, and the production and secretion of at least three hemopoietic regulatory factors (erythropoietin, colony stimulating factor and a multipotential stem cell enhancing and maintaining factor) into the extracellular fluid of bone marrow-derived macrophages grown on hydrophobic teflon foils, a hypothesis for the regulation of hemopoiesis is proposed. The macrophage is viewed as that component in the hemopoietic microenvironment responsible for hemopoietic regulation, by acting as an "intermediary" between extracellular signals and the controlled release of regulatory factors acting on responsive cells at different levels in the hemopoietic hierarchy.