Stimulation of haemopoietic stem cell proliferation: characteristics of the stimulator-producing cells.

Media conditioned by regenerating murine bone marrow cells contain a stimulator of haemopoietic stem cell proliferation. Fractionated cell populations have been examined for production of this stimulatory activity in order to characterize its cellular source. The stimulator is produced by adherent, phagocytic, radioresistant, Thy 1.2-, Fc+ cells in a population concentrated in a density range of 1.064-1.072 g/ml. The results indicate that the producer cells reside in the heterogenous mononuclear phagocytic population of the bone marrow.