Fig. 1. Notch activation in the mouse cardiac cell lineage. (A-D) Notch1 expression revealed by whole-mount in situ hybridization at E10.5. In wild-type embryos (A,B), it is difficult to detect Notch1 expression in the heart at the whole-mount level, whereas in NICD1-activated hearts (C,D), Notch1 is strongly induced in whole heart which shows an abnormal morphology. The heart regions are magnified (B,D). (E-L) Immunohistochemical analysis of cleaved-Notch1 (NICD1 Ab) at E10.5. NICD1 signals are only observed in the nuclei of endocardial cells in the wild-type heart (E-H), whereas the signals are observed in both endocardial and myocardial cells in NICD1-activated hearts (I-L). The images of other sections were generated to indicate that only endocardial cells are NICD1-positive in the AV cushion of the wild-type embryonic heart (H). Arrows indicate endocardial cells. Arrowheads indicate myocardial cells. Abbreviations: LA, left atrium; RA, right atrium; LV, left ventricle; RV, right ventricle. Scale bars: 200 µm in E,I; 50 µm in F-H,J-L.