Fig. 1. Cardiovascular phenotype of Nrp1^-/- embryos. Dark-field images of freshly dissected yolk sacs from E9.5 embryos with the indicated genotypes (A,E) shows defects in yolk-sac vascular remodeling that are further highlighted by Pecam1 staining of the wild-type (B) and mutant (F) yolk sacs. Angiograms at E8.5, just after the onset of plasma flow (C,G) show that vessels are lumenized in both wild-type (C) and knockout (G) embryos, but that Nrp-1^-/- already show altered yolk sac vessel geometry (G). Pecam1 staining of yolk sacs (D,H) were used to quantitate the phenotype based on the number of branchpoints per mm² (I), average vessel segment length (J) and area of avascular spaces (K) at different stages of development for wild-type and knockout embryos. ^*P<0.05, ^**P<0.01, ^***P<0.005; Student's two-tailed t-test. A, aorta; H, heart; YS, yolk sac. Scale bars: 1 mm in A,E; 500 µm in B,C,F,G; 100 µm in D,H.