Proliferative kinetics and differentiation of murine blast cell colonies in culture: evidence for variable G0 periods and constant doubling rates of early pluripotent hemopoietic progenitors.

Several investigators have described hemopoietic colonies expressing multi-lineage differentiation in culture. We recently identified a class of murine hemopoietic progenitors which form blast cell colonies with very high replating efficiencies. In order to clarify further the relationship between progenitors for blast cell colonies and progenitors for the multilineage hemopoietic colonies in culture, we carried out analyses of kinetic and differentiation properties of murine blast cell colonies. Serial observations of the development of blast cell colonies into multilineage (and single lineage) colonies in cultures of spleen cells obtained from 5-fluorouracil (5-FU)-treated mice confirmed the transitional nature of the murine blast cell colonies. The data also suggested that the early pluripotent progenitors are in G0 for variable periods, and that when triggered into cell cycle, they proliferate at relatively constant doubling rates during the early stages of differentiation. The notion that some of the pluripotent progenitors are in G0 was also supported by long-term thymidine suicide studies in which spleen cells were exposed to 3H-thymidine with high specific activity for 5 days in culture, washed, and assayed for surviving progenitors. Comparison of replating abilities of day-7 and day-16 blast cell colonies from normal as well as 5-FU-treated mice indicated that some of the day-7 blast cell colonies are derived from maturer populations of progenitors which are sensitive to 5-FU. In contrast, progenitors for the day-16 blast cell colonies are dormant in cell cycle and were not affected by 5-FU treatment. Previously we reported that progenitors for day-16 blast cell colonies have a significant capacity for self-renewal. These observations suggest the hypothesis that the capability for self-renewal is accompanied by long periods of G0, and that once commitment to differentiation takes place, then active cell division occurs.