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Fig. 6 Overexpression of Grem2 induces ectopic atrial myocardium. (A) Lateral view of live wild-type (WT) and grem2 mRNA-injected (15 pg) cmlc2-egfp transgenic embryos (grem2 OE) at 48 hpf shows ectopic cardiac tissue (arrowhead). Anterior-posterior (A/P) and dorsal-ventral (D/V) axes are indicated. (B) In situ hybridization shows ectopic sites expressing gata-5 along the paraxial mesoderm (arrowheads). (C) WT zebrafish embryos were injected with grem2 mRNA at the 1-4 cell stage; non-injected embryos served as controls. At 24 hpf, Grem2 overexpression (OE) led to enlarged or ectopic (arrowhead) amhc expression in 100% of injected embryos, whereas vmhc expression was reduced (arrow) or absent in 54.4%. These results remained consistent at 48 hpf, with 96% of embryos displaying ectopic amhc expression (arrowhead), and 52.2% displaying reduced vmhc expression (arrow). Insets display enlarged frontal images. Normal atrium (A) expressing amhc and ventricle (V) expressing vmhc are marked; dorsal (24 hpf) and lateral (48 hpf) views; anterior is to the left in all panels.