CFU-S activity within LSKFlt3hi cells resides in the Mplhi, not in the Mpl−, subpopulation. (A) Coexpression pattern of Mpl and Flt3 on Kit-enriched, lineage-negative (Lin−), Sca-1+ and Kit+ (LSK) BM cells. Gates denote the sorting strategies used to purify LSKFlt3hiMpl− (Flt3hiMpl−), LSKFlt3hiMplhi (Flt3hiMplhi), and LSKFlt3− cells. Percentages indicate mean quadrant frequencies within LSK cells from 8 experiments. Right panels show typical purity analysis for LSKFlt3hiMpl− and LSKFlt3hiMplhi cells. (B) Number and size distribution of day 11 CFU-S in mice that underwent transplantation with 50 LSKCD34+Flt3− (CD34+Flt3−) cells (n = 20), 250 LSKFlt3hiMplhi cells (n = 37), or 250 LSKFlt3hiMpl− cells (n = 34), respectively. Mean plus or minus the standard error of the mean (SEM) values from 3 experiments. (C) Left and middle panels show photographs and cell morphology (original magnification, ×500) of typical CFU-S colonies in spleens of mice that underwent transplantation with LSKCD34+Flt3− and LSKFlt3hiMplhi cells, respectively. Right panels show spleens and morphology of cells picked from CFU-S from mice that underwent transplantation with LSKFlt3hiMpl− cells. To the left, a typical spleen transplanted with 250 LSKFlt3hiMpl− cells and no CFU-S (31 of 34 mice), and to the right, one of the few cases (3 of 34 mice) in which a small CFU-S was observed in mice that underwent transplantation with LSKFlt3hiMpl− cells. Below, typical cell morphology of cells picked from a spleen without CFU-S (left), and from a small colony (right) derived from LSKFlt3hiMpl− cells.
