Number of colony-forming cells produced by cells sorted on day 4 after 2 divisions and exposed for another 2 days to cytokines ± MS5 cells.

The 2-step procedure is depicted in Figure 1. In panel A, 2-division cells from 4-day cultures with (closed symbols) or without (open symbols) MS5 cells were plated in LTC (50 cells/well). Each point from panel B indicates the number of CFCs produced by 50 cells that had completed a total of 4 divisions by day 6 and originated from the initially slowly dividing population (2 divisions by day 4). Panel B1 corresponds to cells cultured for 4 days without MS5 and then from day 4 to day 6 with (closed symbols) or without (open symbols) MS5 cells. Panel B2 corresponds to cells cultured for the first 4 days with MS5 cells. All cultures contained the same cytokines. Comparison of the number of CFCs generated by 4-division cells with or without MS5 yielded highly significant differences (*P < .01)