Fig. 7. Nodal^600/600 embryos exhibit heart and lung situs defects. In wild-type (WT) embryos at 9.5 dpc (A), the heart loops laterally across the body of the embryo, while in Nodal^600/600 mutant embryos (B), the heart loops ventrally. Hearts are visualized by expression of the myocardial marker -cardiac actin. Looping differences are summarized in A’ and B’. (C) In WT 12.5 dpc embryos (left), the lungs branch to give four lobes on the right and one lobe on the left. By contrast, Nodal^600/600 mutant embryos (right) have four right lobes and two or three left lobes, indicating partial right isomerism. In WT embryos, the apex of the heart (H) points to the left, while in mutant embryos it is more medially positioned. These differences are summarized in the cartoon in C’. Lung lobes are labeled R1, 2, 3 and 4 to indicate the cranial, middle, caudal and accessory lobes, respectively, and L1, 2 and 3 to indicate the normal left lobe or duplicated cranial lobe (L1), the duplicated middle lobe (L2) and the duplicated caudal lobe (L3). Hearts from WT (D,E) and Nodal^600/600 mutant (F,G) embryos collected 12 hours before birth. The thoracic cavity has been dissected away to reveal the heart and the major blood vessels leading from it. The view is from the ventral side of the embryo in D and F, and of the same embryos rotated onto their right sides in E and G. In all panels, rostral is upwards. (D) WT heart. The aorta (ao) emerges from behind the pulmonary artery (pa) and arches to the left. The subclavian and common carotid arteries ascend from the aortic arch. The pulmonary artery emerges form the heart and at this stage of development empties through the ductus arteriosus (indicated by *) into the aorta. The left and right pulmonary arteries that deliver blood to the lungs carry very little blood prior to birth and are not visible in these embryos. (E) View of embryo in D from the left hand side. The pulmonary artery (pa) can be clearly seen to emerge from in front of the aorta (ao). (F) Nodal^600/600 mutant embryo. A large aorta (ao) is present in this embryo that divides into four ascending vessels. The pulmonary artery is not visible from this angle lying directly behind aorta. (G) View of embryo in F from the left-hand side. In addition to the aorta, from this angle it is possible to see the pulmonary artery (pa) that empties into the aorta through the ductus arteriosus (indicated by *). (D’-G’) Schematics of D-G, indicting the position of the aortic arch and ascending arteries in red and the pulmonary artery in blue.