Alk1 deletion resulted in abnormal extraembryonic vasculature in E16.5 L1cre(+);Alk1^3loxP/3loxP fetuses. (A,B) Gross morphology of control and L1cre(+);Alk1^3loxP/3loxP mutant fetuses enclosed in the yolk sac attached to the placenta (PL). Note bulged arteries (A) and veins (V) in the mutant yolk sac. The inset in panel B shows magnified view of typical dilated, tortuous vitelline vessels in the mutants. (C,D) Umbilical arteries (UA) and veins (UV) are connected to the placenta, whereas vitelline arteries (VA) and veins (VV) are connected to the yolk sac. Note markedly enlarged VA and AVMs (circled; see enlarged view in Figure 5D) in the mutants. (E,F) Cross-sectional view of the extraembryonic vessels indicated by the scissors symbols in panels C,D demonstrates marked dilation and thinning of mutant VA (F), which has a similar morphology as control VV (E).