Comparison of megakaryocyte colony numbers and thrombopoiesis in c-myb–null and combined wildtype and c-myb-heterozygous embryos. (A) The number of MEP (± SEM) in staged wildtype and c-myb-heterozygous mouse embryos () compared with the number of Meg-CFC (± SEM) in similarly staged c-myb–null embryos (▭). Data derived from Figures 1C and D. YS = yolk sac, FL = fetal liver. (B) Total platelet numbers (± SEM) in c-myb–null (−/−, dashed line) and combined wild-type (+/+) and c-myb-heterozygous (+/−) (solid line) mouse embryos at E12.5 and E15.5 of mouse gestation. Platelets and blood cells, obtained from 3 to 5 μL of whole blood diluted in PBS/0.1M EDTA/0.1 mg/mL heparin (Sigma-Aldrich, St Louis, MO), were counted on a hemacytometer at 40× magnification using phase optics. Total platelet counts were based on platelet concentration × total blood cell measurements.¹⁶