Flt-1^−/−ES cell cultures have increased vascularization.

Day 8 differentiated flt-1^−/− (A-C),flt-1^+/− (D-F), and WT (G-I) cultures were processed for β-galactosidase detection (A,D,G) or reacted with an antibody to PECAM (B,E,H). A and B show one quadrant of the relatively large β-galactosidase⁺ (A) or PECAM⁺ (B) sheet of cells that characterizes the flt-1^−/−phenotype. In contrast, an extensive vascular plexus is found in bothflt-1^+/− (E) and WT (H) ES cell cultures. Arrowheads (A) outline an intensely stained β-galactosidase⁺ ring of cells that surrounds most of the β-galactosidase⁺ cells. Arrows (A,B) point toflt-1^−/− vasculature that looks WT. (C,F,I) Phase contrast images of PECAM-labeled fields in B,E,H. Magnification is ×10.