Outbred Ekkwill strain (EK) or EK/AB mixed background zebrafish 6–12 months of age were used for ventricular resection surgeries as described previously4 (link). All transgenic strains were analyzed as hemizygotes; details of their construction are described in the separate Methods section. Animal density was maintained at ~4 fish/liter in all experiments. 4-hydroxytamoxifen (4-HT) (Sigma) dissolved with ethanol (5 mg/ml) was diluted in water to 0.5 mg/ml for intraperitoneal injections. 10% ethanol was used as a vehicle control. EGFP labeling quantification is described in the separate Methods section. Heat-shock experiments were performed as described previously27 (link), using double transgenic hsp70:dnfgfr1; cmlc2:nucDsRed2 or hsp70:dnfgfr1; gata4:EGFP animals. For BrdU incorporation experiments, 2.5 mg/ml BrdU (Sigma) was injected intraperitoneally once daily for 3 days prior to collection. Immunofluorescence, in situ hybridization, and Acid Fuchsin Orange G stains (detecting fibrin and collagen) were performed as described previously4 (link). Primary antibodies used in this study were: anti-Mef2 (rabbit; Santa Cruz Biotechnology), anti-Myosin heavy chain (F59, mouse; Developmental Studies Hybridoma Bank), anti-β-catenin (rabbit; Sigma), anti-zf Raldh2 (rabbit: Abmart), anti-BrdU (rat; Accurate), and anti-GFP (rabbit, used only for co-detection with BrdU; Invitrogen). Secondary antibodies (Invitrogen) used in this study were Alexa Fluor 594 goat anti-rabbit IgG (H+L) for anti-Mef2, Alexa Fluor 594 goat anti-mouse IgG (H+L) for F59, Alexa Fluor 594 goat anti-rat IgG (H+L) for anti-BrdU, and Alexa Fluor 488 goat anti-rabbit IgG (H+L) for anti-GFP. In situ hybridization and immunofluorescence images were taken using a Leica DM6000 microscope with a Retiga-EXi camera (Q-IMAGING), and confocal images were taken using a Leica SP2 or SP5 confocal microscope. Physiology methods are described in the separate Methods section.