Branchial arch artery defects in MRTF-B mutant embryos. (A–D) Defects of the great vessels in E13.5 mutant embryos. In the transverse section of the wild-type embryo, the pulmonary trunk communicates with the descending aorta via the ductus arteriosus (A). Mutant embryos display hypoplastic pulmonary trunk (arrowhead) and no ductus arteriosus (B), persistent right-sided ductus arteriosus (C), and interrupted aortic arch (D). Note the abnormal shape and position of the descending aorta where the ductus arterious joins (*). a, aorta; at, atrium; d, ductus arteriosus; rv, right rentricle. (E–H) Schematic diagrams of aortic arch defects seen in A–D. Dotted lines represent normal regression. Black areas depict abnormal regression, whereas blue areas indicate abnormal persistence. (E) Patterning of the branchial arch arteries in the wild-type embryo with normal regression of the right sixth artery (-R6). (F) Absence of the ductus arteriosus resulting from abnormal regression of the left sixth artery (-L6). (G) Persistent right-sided ductus caused by abnormal regression of the left sixth artery (-L6) and abnormal persistence of the right sixth artery (+R6). (H) Interrupted aortic arch and loss of both carotid arteries, due to abnormal regression of the left fourth (-L4) and both right and left third arteries (-R3, -L3). R3, R4, and R6, right arch arteries 3, 4, and 6, respectively. L3, L4, and L6, left aortic arch arteries, respectively.