RA signaling regulates the development of hemogenic endothelium during murine embryogenesis. (A) Phenotypic characterization of the SP compartment in E9.5 WT, and (B) Raldh2^−/− yolk sac. Yolk sacs were stained with Hoechst 33 342, c-Kit-APC, Flk1-PE and CD45-FITC antibodies for visualization and phenotypic analysis of the SP compartment by flow cytometry. Cells were additionally stained with PI to enable exclusion of nonviable cells. (C) Quantitative analysis of the SP compartment in E9.5 Raldh2^−/− versus WT yolk sac, calculated as a percentage of the total live cell population plus or minus SEM (n ≥ 3). Statistical analysis of significance was determined by Student t test, with a confidence interval of 95% (P ≤ .05).