Hemopoietic stromal precursors in long-term culture of bone marrow: I. Precursor characteristics, kinetics in culture, and dependence on quality of donor hemopoietic cells in chimeras.

The adherent cell layer of long-term Dexter type bone marrow culture contains hemopoietic stromal precursors capable of transferring the hemopoietic microenvironment on transplantation in vivo. The foci of ectopic hemopoiesis that are created correspond to normal bone marrow in the character of the maintained hemopoiesis. Radiosensitivity of the cultured stromal precursors does not differ from preculture sensitivity. Stromal precursors lose osteogenic potential after 5-6 wk in culture, but are capable of creating stable foci of ectopic hemopoiesis composed of up to 10(8) nucleated hemopoietic cells. These results prove that bone is not an essential component of the hemopoietic environment in ectopic foci. In chimeras exposed to equal doses of radiation but reconstituted by freshly isolated or repeatedly passaged bone marrow, the content of stromal precursors is lower when hemopoietic cells of diminished self-maintenance are used. These data suggest the existence of a hemopoietic stromal precursor hierarchy with at least two levels of precursor differentiation.