mDll4 overexpression causes morphogenetic defects in the developing heart. PECAM1 immunostaining of cryosections from E9.5 WT and DT embryos (A) WT heart ventricle (v) showing normal trabeculation. (B) DT heart ventricle (v) revealing reduced trabeculation. (C) WT heart atria (a). (D) DT heart atria (a) revealing gross hypertrophy. (E) Outflow tract of heart from WT embryo. (F) Outflow tract of heart from DT embryo, showing acellularization of the outflow tract cushions (*). SMA immunostaining of cryosections from E9.5 WT and DT embryos (G) heart ventricle (v) of WT embryo. (H) Heart ventricle (v) of DT embryo showing aggregation of cardiomyocites at the periphery of the ventricle and irregular distribution of cardiomyocites through the trabeculations, relative to WT. Whole-mount PECAM1 immunostaining of E9.0 WT and DT embryos. (I) WT embryo imaged at the heart level, showing normal arteries at the first (green arrow) and second branchial arches (blue arrow; DA, red arrow). (J) DT embryo imaged at the heart level, showing the presence of a fully formed artery at the first branchial arch (blue arrow) but only a very reduced or nonexistent artery at the second branchial arch (green arrow; DA, red arrow).