Alk1 deletion results in multiple AVM formations. Dissection microscopic views of representative arteries (A) and veins (V) in control (A,C,E,G) and L1cre(+);Alk1^3loxP/3loxP (B,D,F,H) E16.5 embryonic yolk sacs. In the control yolk sac (A,C), arteries and veins are intercalating and not connected directly to one another. In the mutants (B,D), however, there are numerous regions where dilated and tortuous arteries and veins are directly connected without the connecting capillaries. (E-H) India ink was injected into the vitelline artery to visualize the yolk sac vessels. In control (E,G), arteries and veins were easily distinguishable and were connected by capillaries. In mutants (F,H), numerous AVMs (indicated by arrows) were formed between arteries and veins.