Apoptosis and proliferation of wild-type and p85α^-/- c-kit⁺ fetal liver cells. (A) Apoptosis of wild-type and p85α^-/- c-kit⁺ or ter119⁺ day-14.5 fetal liver cells. Freshly isolated fetal liver cells were stained with either annexin V–PE or c-kit–FITC antibodies to identify the percentage of c-kit⁺ cells (top row) or annexin V–FITC and ter119-PE antibodies (bottom row) to identify the percentage of ter119⁺ cells undergoing apoptosis by fluorescence cytometry in vivo. The numbers in each quadrant represent the percentage of total cells in that quadrant. A representative dot plot for each genotype is shown. Four other experiments showed similar results. (B) Proliferation of wild-type (▦) and p85α^-/- (▨) c-kit⁺ day-14.5 fetal liver cells in response to no growth factors, Epo and KitL in combination, or alone. Freshly isolated c-kit⁺ fetal liver cells were plated in 96-well plates in replicates of 6 in the presence of 5% fetal calf serum with no additional growth factors or 10 ng/mL KitL and 5 U/mL Epo in combination or alone. After 48 hours of culture, cells were pulsed with tritiated thymidine and harvested 16 hours later for measurement of β emission. Results represent the mean thymidine incorporation ± SEM of 5 independent experiments. *P < .05 by Student paired t test.