Fig. 2. Investigating causes of defective flow in Nrp1^-/- embryos. (A) In situ hybridization with an Nrp1 antisense riboprobe on a section from a wild-type embryo at 10 somites shows expression in the atrium (marked A) and not the ventricle (marked V), as well as expression in the dorsal aortae (arrows) and yolk-sac vessels (arrowheads). (B,D) Serial sections of E8.5 wild-type (B) and mutant (D) embryos stained for Pecam1 show that dorsal aortae (DA) are present and lumenized in both cases, though often collapsed in the mutant (D). (C) By E9.5, collagen IV staining highlights the dorsal aortae (DA), gut (G) and heart (H) of wild-type embryos. (E) In mutant embryos, one or both dorsal aortae are often missing (arrow). (F,G) Intersomitic vessel sprouting is visible in E9.5 wild-type embryos stained for Pecam1 (F) but is delayed in mutant embryos (G) and also shows the absent dorsal aorta (arrow). (H,I) The cephalic region of E9.5 wild-type embryos shows an extensive vasculature (H) that is also present in the mutant embryos (I); however, the vasculature of the mutants is more sparse and many vessels do not appear to be lumenized. The internal carotid artery (ICA) has started forming in the wild-type (H) but is lacking in the mutant (I). NT, neural tube. Scale bars: 50 µm in A,B,D; 100 µm in F-I; 200 µm in C,E.