Fig. 2. Misexpression of Hesr1 and Hesr2 in the entire embryonic mouse heart at E9.5. (A-E) The expression of Hesr1 (A,E) and Hesr2 (B,D) was examined by whole-mount in situ hybridization of double-transgenic mouse embryos. The embryos were produced by crossings between Mesp1-Cre and either CAG-lox-CAT-lox-Hesr1 (A,D) or CAG-lox-CAT-lox-Hesr2 (B,E) mice. (C) Expression levels of Hesr1 and Hesr2 in two independent lines for each gene derived from these misexpressing mouse embryonic hearts (n=10). Both Hesr1 and Hesr2 are strongly induced (2.5-5.5 fold) compared with the wild-type heart. (F-H) Sections stained with Hematoxylin and Eosin showing a normal morphology for the atrium, AV canal (bracket), and the ventricle in the wild-type embryonic heart (F). The width of the AV canal (bracket) is smaller in the Hesr1-ME heart (G). The atrium and ventricle seem to be directly connected (arrowhead) in the Hesr2-ME heart (H). (I-K) Analysis of cardiomyocytes in the left ventricles of the mouse embryonic heart by transmission electron microscopy. Myofibrils were observed to be thin and disorganized in the Hesr1-ME (J) and Hesr2-ME (K) hearts, compared with wild type (I). In the Hesr2-ME hearts, Z bands (arrowhead) were sometimes barely discernible (asterisks). Scale bars: 100 µm in F-H; 1 µm in I-K. RA, right atrium; RV, right ventricle; LA, left atrium; LV, left ventricle.