CSF1R deficiency impairs the emergence of ProB cells in the fetal liver. (A) (Left) Representative FACS profiles of ProB cells in E13.5 FL of Csf1r^+/+ (Wt) and Csf1r^−/− littermate embryos (3 experiments). Numbers represent mean percentage of gated populations relative to total FL cells. (Right) Mean percentage (± SEM) of ProB cells in E13.5 FL of Csf1r^+/+ (light gray) and Csf1r^−/− littermate embryos (black) (n = 12-13 embryos per genotype). (B) Mean percentage (± SEM) HSCs, LMPPs, and CLPs (Lin^–KIT^lowSCA-1^lowIL7R⁺FLT3⁺) in E13.5 FL of Csf1r^+/+ and Csf1r^−/− littermate embryos (n = 6-12 embryos per genotype, 2-3 experiments). (C) Lethally irradiated adult Wt recipient (CD45.1) mice were transplanted with 400 000 unfractionated Csf1r^−/− (CD45.2) or Csf1r^+/+ littermate (CD45.2) E14.5 FL cells together with 400 000 unfractionated Wt CD45.1 competitor E17.5 FL cells. (Left) Representative FACS profile of CD45.2/CD45.1 distribution within ProB, PreB, or IgM⁺ B cells (Lin^–B220⁺CD19⁺CD43^–IgM⁺cells) in BM from the different genotypes at 7 weeks post-transplantation. (Right) Mean percentage (± SEM) of CD45.2 cells within the indicated population in mice transplanted with Csf1r^+/+ and Csf1r^−/− littermate embryos (n = 7 mice per genotype with 4 to 5 different donor embryos per genotype). (D) Lethally irradiated adult Wt recipient (CD45.1) mice were transplanted with 5 × 10⁶ unfractionated Csf1r^−/− (CD45.2) or Csf1r^+/+ littermate (CD45.2) E14.5 FL cells. Mean number (± SEM) of donor-derived ProB, PreB, and IgM⁺ B cells in BM reconstituted with Csf1r^+/+ or Csf1r^−/− littermate embryos at 16 weeks post-transplantation (n = 7 mice per genotype with 4 different donor embryos per genotype). Statistical significance was tested between Csf1r^+/+ and Csf1r^−/−.*P < .05. ns, not significant.