PE migration occurs through a cellular bridge to the heart. Lateral views of zebrafish hearts at 72 hpf. (A) Brightfield image of a live heart (n = 10). Arrow indicates the PE. (B) H&E stained section through heart and pericardium (n = 5). Arrow indicates the PE. (C-D) Confocal images of whole-mount fixed zebrafish. Epicardial cells marked with immunostaining for DsRed2 (red), which is driven by the tcf21 promoter. Nuclei are stained with DAPI (blue) and cardiomyocytes are marked with activated cell adhesion molecule (ALCAM; green). (C) The PE, which is outlined, forms a bridge between the ventricle and the pericardial wall (n = 10). (D) Magnified Z-stack projection and orthogonal slice of area boxed in C. Orthogonal slice at line indicated by “x” shows cross-section of cells below the line. White arrows indicate cells within the PE cluster that are not expressing tcf21. For all panels, anterior is to the left and V is ventricle. Scale bars = 50 microns.