Studies on the kinetics of the erythroid colony-forming cell

The appearance, rate of formation, and morphologic characteristics of the erythroid colony-forming cell (CFU-E) was studied in control, exhypoxic polycythemic, and anemic mice. Polycythemia resulted in a significant reduction in CFU-E number and a lag time of approximately 8 h before colonies were visible in culture. Thereafter, the rate of formation of colonies, as assessed by the slope of the increase in their number, was identical to the control. The slower appearance of CFU-E in polycythemia was corrected by the injection of erythropoietin (Ep) 8 h prior to sacrifice, but an absolute increase in colony number only occurred when Ep was injected 24 h previously. Polycythemia also slowed the rate of colony maturation, so that at later culture times the colonies more frequently contained fewer cells and consisted of basophilic normoblasts. Anemia did not shorten the time of first appearance of colonies, but their rate of maturation was significantly more rapid than the control. These findings are best explained by a redistribution of CFU-E in various phases of the cell cycle when erythropoiesis is manipulated. Unit-gravity sedimentation studies suggested a greater fraction of CFU-E in long G1 or G0 in polycythemia. In addition, the kinetics of CFU-E development were identical to polycythemia in mice in which S and post-S-phase CFU-E were removed by hydroxyurea injection. It is likely that Ep stimulates CFU-E in all phases of the cell cycle. After stimulation by Ep, however, CFU-E in long G1 must move through the cell cycle prior to commencing cell division. This transit time of 8 h, which is unaffected by the Ep concentration in the medium, results in a lag in appearance and maturation of colonies derived from these cells. These results demonstrate that cellular variables are important in CFU-E kinetics in vitro and provide insights into factors affecting the regulation of normal erythropoiesis.