Fig. 2. Developmental changes of ventricular activation sequence and the expression of Cx40 and ECE1. (A-C) Isochronal maps of action potential propagation recording on the dorsal ventricular surface of looping, and septating stage hearts. The first activation site is indicated in red. Developmental stages and time resolution of each frame are also indicated. (D) Topological shift of ventricular activation sequences from the immature to the mature pattern during embryogenesis is presented as % of hearts exhibiting the apex-to-base pattern. More than 12 hearts examined at each time point. (E-G) Whole-mount in situ hybridization for Cx40 mRNAs (positive signals indicated by white arrows) in hearts cut to half frontally at stages indicated. Insets are controls stained with sense probes. (H) A transverse section of E8.5 heart stained for Cx40. (I) High power view of the boxed area in H. (J-L) As in E-G but for ECE1 mRNAs. (M) As in H but for ECE1. (N) High power view of the boxed area in M. at, atrium; avj, atrioventricular junction; ec, endocardium; ivs, interventricular septum; lv, left ventricle; ot, outflow tract; rv, right ventricle; tb, trabeculum.