Fig. 2. (Top) Positions of sections A-O. (A-E) Serial sections of normal heart. The pulmonary artery and aorta are connected to the right and left ventricles, respectively. Both right and left ventricles develop normally with the extensive trabecular formation and a ventricular septum. Right and left atrio-ventricular canals are formed at the correct position. (F-J) An abnormal heart in which Tbx5 was misexpressed ubiquitously (Type 1). Ventricular septum formation was severely disturbed, resulting in a single ventricle. The ventricular wall was thin, and the trabeculae were coarse and rough. Both right and left atria were dilated with an atrial septum defect (ASD; arrowhead in I). The aorta and pulmonary artery were fused at their base and connected to the single ventricle, resulting in a double outlet left ventricle (DOLV; arrowhead in F). Arrowhead in J indicates the atrio-ventricular canal. (K-O) Another type of malformation was observed (Type 2) in which the left ventricle expanded and the right ventricle shrank. The relative sizes of these two ventricles indicate a shift of the ventricular septum formation to the right, although the trabecular formation and the thickness of ventricles were not affected. Conal septation/rotation defects were also observed (arrowhead in M and N). The atrial septum formed, but it was thin and membranous. (P) Illustrations of the induced malformations. Type 1: atrial and ventricular septum defects (ASDs and VSDs). Alterations in both the aorta and pulmonary artery (DOLV and conal septation/rotation defects). Thin ventricular wall, suggesting abnormal differentiation of cardiac muscle cells. Dilatation of atrium. Type 2: VSD with a shift of the position of septum, resulting in a small right ventricle and expanded left ventricle, the conal septation/rotation defects with coarctation and thin atrial septum.