Defective heart development in Alk5-cKO mutants. Transverse sections of an E12.5 control (a–c) and Alk1GFPCre/+;Alk5f/f mutant (d–f) stained with anti-αSMA antibodies (brown color). (a–c) Section of a control embryo show normal development of heart with atrioventricular (AV) cushion, interventricular septum, and trabeculation. Magnified views of dorsal aorta (DA) and AV cushion areas are shown in (b) and (c), respectively. (d–f) Abnormal heart development in Alk5-cKO embryos. Magnified views of DA and AV cushion areas are shown in (e) and (f), respectively. Endocardial cushions are found in an area where the right ventricle should be located, and the ventricular septum did not develop properly. Development of DA is unaffected. Inset in (e) show αSMA-Ab stained DA in the subsequent section of the same Alk5-cKO embryo. Abbreviations: Es, esophagus; IAC, inferior AV cushion; IVS, interventricular septum; Lg, lung; LA, left atrium; LV, left ventricle; RA, right atrium; RLAC, right lateral AV cushion; SAC, superior AV cushion.