Zebrafish labelled with fli:GFP, showing vasculature and beating heart endocardium. Due to the depth of cardiac tissue at early stages, only the superficially located venous pole/future atrium of the heart tube can be imaged with sufficient resolution. To image arterial pole/ventricle development, we surgically removed the anterior region (anterior to the otic placode) of embryos with N.5SF Dumont forceps 1 hour before the desired stage

After surgery, undamaged posterior regions (identified by an intact yolk sac, undamaged body and beating heart, Supplementary Video 1), which we refer to as body explant (BE) fish, were transferred to supplemented L15 medium for an hour (15% fetal bovine serum, 0.8 mM CaCl2, 50 μg/ml penicillin, 0.05 mg/ml streptomycin, 0.05 mg/ml gentamicin), as previously described for ex-vivo heart cultures. After an hour of recovery, healthy BE fish which show closed systemic circulation