Fig. 3. Brg1^fl/fl:Tie2-Cre^+/0 blood cells are largely depleted by E9.5. Mutant embryos were crossed onto the ROSA26 reporter line and were stained with X-gal for detection of β-galactosidase, which marks Cre-mediated excision events. (A,B) Blood vessels from Brg1^fl/+;R26R^R/+:Tie2-Cre^+/0 control (A) and Brg1^fl/fl;R26R^R/+:Tie2-Cre^+/0 mutant (B) E8.5 yolk sacs contain a comparable number of blood cell precursors, the majority of which express Tie2-Cre. (C) The majority of circulating blood cell precursors still express Tie2-Cre at E9.5 in Brg1^fl/+;R26R^R/+:Tie2-Cre^+/0 control embryos. (D) By contrast, fewer circulating blood cell precursors are found in Brg1^fl/fl;R26R^R/+:Tie2-Cre^+/0 mutant embryos at E9.5, but, of those cells that persist, the proportion of cells that express Tie2-Cre and are presumably deficient for Brg1 expression is dramatically decreased when compared with the blood cells in the control embryo (C). The arrow in the inset points to an abnormally shaped mutant (Brg1^fl/fl:Tie2-Cre^+/0) blood cell. Scale bar: 40 µm. (E) Mean percentages of Tie2-Cre-positive (β-galactosidase-positive) blood cells from multiple serial sections of two control and two mutant embryos at E8.5, and two control and two mutant embryos at E9.5, carrying the ROSA26 reporter, as detected by X-gal staining. A total of 941 and 487 blood cells were counted from control and mutant sections at E8.5, respectively, while a total of 1,656 and 893 blood cells were counted from control and mutant sections at E9.5, respectively. Errors were calculated as s.e.m.