A stimulator of stem cell proliferation in regenerating bone marrow.

A factor, capable of stimulating resting haemopoietic stem cells into DNA-synthesis has been extracted from regenerating bone marrow. It has a molecular weight in the range of 30.000-50.000 daltons and is not detectable in normal bone marrow. Used in combination with a stem cell proliferation inhibitor, previously described, it will restimulate proliferation in stem cells initially stopped by the inhibitor. Conversely, stimulation produced by this factor can be reversed by the addition of inhibitor. It is concluded that stem cell proliferation is controlled by an appropriate balance of endogenously produced stimulatory and inhibitory factors.