Stem cell number versus the fraction synthesizing DNA.

It has been widely held that the fraction of spleen colony-forming units (CFU-S) in DNA synthesis is inversely correlated with CFU-S numbers. In 750 measurements the expected negative slope of the linear regression of CFU-S in S-phase on CFU-S was found to be significant only when the measurements from irradiated mice were pooled. In contrast, a significant positive correlation was observed when the measurements from normal controls were pooled. A hypothesis is offered to account for the variable relationship between CFU-S and CFU-S in S. The hypothesis fully recognizes the crucial role of both parameters in normal and regenerative hemopoiesis.