Serial block face scanning z-stacks through a wt1b-overexpressing heart at 5 dpf. Serial stacks through a ventricle from a myl7:Gal4;eGFP:UAS:wt1b embryo at 5 dpf. Images were obtained by serial block face scanning electron microscopy. Note the absence of a compact and organized myocardial layer and the enlarged cardiac jelly separating the endocardium and the myocardium. Also visible the extensive areas filled with extracellular matrix. EpC, epicardial cell, EnC, endothelial cell, Ery, erythrocyte, CM, cardiomyocyte, ECM, extracellular matrix, v, ventricle, at, atrium. Scale bar, 20 μm.