The endocardial expansion in wkm mutants is bmp4 driven. (A-B′) ISH for zebrafish has2 in wkm mutants and siblings injected with p53 control MO (A) or p53 MO + bmp4 MO at 52 hpf and examined at 2 dpf. Dashed lines outline the morphology of the heart. Arrowhead indicates the location of the AVC. (C-F′) Endothelial Kdrl:GFP (C-D′) or endocardial Alcama (E-F′) expression in wkm mutants and siblings injected with p53 MO or p53 MO + bmp4 MO. (G) Model of AVC boundary regulation. In zebrafish, Bmp4 can drive EC primordium development in the AVC. Tmem2 restricts EC formation to the AVC by inhibiting Bmp4. A, atrium; AV, atrioventricular; EC, endocardial cushion; V, ventricle.