Definitive hematopoiesis of embryos at E10.5. (A-D) Quantitative analysis of definitive hematopoiesis at E10.5 by determination of number of CFUs in (A) yolk sac; (B) blood; (C) embryonic remnants (embryonic tissue after removal of yolk sac and amniotic membranes, blood, fetal liver, and tail bud, used for genotyping); and (D) fetal liver. Data represent means (± SD) of 8 or more F2 embryos for each genotype, *P < .01, TGRac1^Flox/null versus all other genotypes. (E,F) Histologic analysis of the aortic clusters at E10.5. Punctuated squares delineate the area shown at higher magnification in the upper-right corner. The figures are representative of all animals analyzed for each genotype. Data are summated in Table 2. Arrow in panel E shows hematopoietic cells on the ventral wall of dorsal aorta. Arrowheads in panel F show circulating primitive red blood cells. No TGRac1^Flox/null mice demonstrated aortic clusters. Images acquired as described in “Light microscopy,” with a 40×/0.5 numeric aperture objective.