Fig. 4. Histology sections through the hearts at E12.5, E14.5 and E16.5. (A-D) Transverse sections through E14.5 wild-type (A) and Bmpr2^E2/E2 (B-D) hearts show a common origin to the aorta (Ao) and pulmonary trunk (PT) and total absence of semilunar (SL) valves (B), while the valves of the atrioventricular canal develop normally (arrowhead in C). A ventricular septal defect is associated (arrowhead in D). The left subclavian artery was occasionally retro-esophageal (arrowhead in B). Oe, Esophagus. T, Trachea. (E-H) Sagittal sections through E16.5 wild-type (E,G) and mutant (F,H) hearts show the aorta (E,F) and the pulmonary trunk (G,H) exiting the same ventricle in the mutants. The pulmonary trunk is the main vessel connecting to the descending aorta (DAo), as a result of the hypoplasia of the aortic isthmus. Note that the thymus (Th) is of normal size in the mutant. (I-N) Series of frontal sections (dorsal to ventral) of an E12.5 wild-type (top panels) and a mutant (lower panels) heart show that, posteriorly, ventricular septation is normal (I), but at the base of the pulmonary trunk, a large ventricular septal defect is visible (black arrow in J). One of the conal cushions appears to form correctly (arrowhead in mutant, K) but most conal tissue (asterisk in wild type, J) is already missing (K). The outflow tract starts septating into two arteries at about the same level that the semilunar valves form in the wild type (L), and aorta and pulmonary trunk are fully septated and in normal relative position beyond that point (M). The aorta branches into the right subclavian artery, while the pulmonary arteries (PAs) arise normally from the pulmonary trunk (N).