Fig. 5. Normal conduction system morphology in Cx40^–/– mice. ß-Galactosidase expression in minK^lacZ/+ (A,C,E) and Cx40^–/–/minK^lacZ/+ (B,D,F) mouse hearts. In the atrioventricular canal, a well-defined atrioventricular node (arrow) and atrioventricular bundle were observed in both minK^lacZ/+ (A) and Cx40^–/–/minK^lacZ/+ (B) mouse hearts. The atria were removed and the atrioventricular canal was viewed from the superior/posterior, with the tricuspid annulus on the right and the mitral annulus on the left. A well-formed left bundle branch was found in both minK^lacZ/+ (C) and Cx40^–/–/minK^lacZ/+ (D) mouse hearts. The left ventricular free wall and mitral valve were removed and the left interventricular septum viewed from the left. A well-formed right bundle branch was present in minK^lacZ/+ (E) and Cx40^–/–/minK^lacZ/+ (F) mouse hearts. The right ventricular free wall and tricuspid valve were removed and the right interventricular septum viewed from the right. Comparable ß-galactosidase expression and morphology were demonstrated in 12/12 minK^lacZ/+ and 10/10 Cx40^–/–/minK^lacZ/+ mice.