Alk1 deletion resulted in abnormal pulmonary vasculature in E17.5 L1cre(+);Alk1^3loxP/3loxP fetuses. Transverse sections of the left lobe of the control (A,C,D) and mutant (B,E,F) lungs. Blood vessels are readily identifiable by the red blood cells in them. Dissection microscopic views of the left lung are shown as insets. The control lung displayed organized vascular trees (A, inset), whereas the mutant lung exhibited dilated, tortuous, and irregular blood vessels (B, inset, arrows). In the control lungs (A,C), bronchial trees and blood vessels are coordinated, and blood vessels are well defined as a circular shape. Br indicates bronchus; PA, pulmonary artery; and H&E, hematoxylin and eosin. In the mutant lungs (B,E), the bronchial lumens are not expanded as much as control lungs, and blood vessels are noticeably enlarged and irregular, presumably resulting from fusions between neighboring vessels (arrowheads in E). (D,F) Immunostaining with anti-αSMA antibodies revealed thinning of blood vessel walls with irregular thickness of smooth muscle layers (arrowheads in F).