Maintenance of hemopoietic stem cells in diffusion chamber cultures.

When murine bone marrow cells are cultured in DC's in irradiated host mice, there is an initial phase of rapid white cell growth followed by prolonged maintenance of steady-state granulopoiesis. This study examined the role of hemopoietic stem cells in this culture system. During the initial growth phase there was a rapid increase in both pluripotential stem cells, CFU-S, and commmitted white cell precursors, CFU-C. The increase in CFU-C was larger, occurred more rapidly, and was biphasic in nature. After day 7 and until at least day 16, the numbers of CUF-S, CFU-C and total white cells were all maintained at relatively constant plateau levels which were typically well below the maximal capacity of the culture system. During the plateau phase the number of CFU-S was slightly greater than the number initially placed in culture, whereas CFU-C had increased by a factor of 2.3 and total white cells by a factor of 10. This appears to reflect a system of growth amplification, with preferential differentiation of CFU-S into CFU-C and thence into the pathway of white cell development. The plateau phase of culture has received very little attention but appears to represent a model, in a closed culture system, of finely regulated granulopoiesis, with an equilibrium between constant numbers of hemopoietic stem cells and their differentiating white cell progeny.