Fig. 3. Abrogation of Alk2 in neural crest cells leads to persistent truncus arteriosus. Hematoxylin and Eosin-stained sections of control (A-F) and Alk2 mutant (G-L) littermate embryos at E14. In controls (A-F), pulmonary artery (PA) and the ascending aorta (Ao) are separated by the conotruncal (aortico-pulmonary) septum, and the right and left ventricles by the interventricular septum (VS). In Alk2 mutants (G-L), the conotruncal septum fails to form, i.e. they display persistent truncus arteriosus (TA; G,H), which is always associated with a ventricular septal defect (VSD, K). Both the atrioventricular and semilunar valves appear normal. An approximate plane of the shown sections is indicated on the schematic drawings (left). Scale bar: 200 µm.