CFU-S content and cycling rate in several strains of mice

The number of CFU-S and the fraction synthesizing DNA have been measured in individual normal mice of several inbred strains. The data gathered during a period of 5 years have been subjected to analysis of variance. Large differences are shown to exist in the number of CFU-S in the femoral bone marrow of individual mice measured on the same day. These differences are greater if measurements are performed on different days. The fraction of DNA synthesizing CFU-S was on average 30% in these normal mice. The range of measurements on both the same and different days was 0%-60%. The large differences in the fraction of CFU-S in S-phase were found even among mice coming from the same cage. This differs from many previous reports where 10% is given as the upper limit for the fraction of CFU-S synthesizing DNA in normal mice. It is suggested that the observed range of individual values from 0%-60% might reflect bursts of proliferation of CFU-S. Irradiation or hydroxyurea administration resulted in disappearance of low and medium values. All mice then had 50%-60% of CFU-S in S-phase.