A Stochastic Model of Self-renewal and Commitment to Differentiation of the Primitive Hemopoietic Stem Cells in Culture

We recently identified a murine hemopoietic stem cell colony which consists of undifferentiated (blast) cells and appears to be more primitive than CFU-GEMM in the stem cell hierarchy. The progenitors for the colony which we termed "stem cell colony" possess an extensive self-renewal capacity and the ability to generate many secondary multipotential hemopoietic colonies in culture. We replated a total of 68 stem cell colonies from cultures of murine spleen cells and analyzed the number of stem cell--and granulocyte(neutrophil)-erythrocyte-macrophage-megakaryocyte (GEMM) colony-forming cells in individual stem cell colonies. Of the 68 stem cell colonies, 35 contained progenitors (abbreviated as "S"-cells) for stem cell colonies. The distributions of S-cells and CFU-GEMM in individual stem cell colonies were extremely heterogeneous. Neither the frequency distributions of S-cells nor CFU-GEMM in stem cell colonies could be fitted well by Poisson distribution. Rather, the frequency distribution of the s-cells could be approximated by a geometric distribution and that of CFU-GEMM by an exponential distribution, both of which are variates of the gamma distribution. Our observations are in agreement with those on the distributions of CFU-S in individual spleen colonies and provided support for a stochastic model for stem cell self-renewal and commitment in culture. Application of the theory of the branching process to the distribution of S-cells revealed a distributional parameter "p" of 0.589 which is also in agreement with the earlier report on the p value for reproduction of CFU-S.