(A–D) Confocal images of sox9b:EGFP expression in embryonic, larval, and adult zebrafish hearts. Cardiomyocytes are marked with activated leukocyte cell adhesion molecule (ALCAM; red in A–C) and nuclei are stained with DAPI (blue); (A) Lateral view of a 24 hpf embryo. The developing heart is outlined and indicated by the arrow. (B,C) Ventral views of a 72 and 96 hpf hearts. Expression of sox9b:EGFP is detected in cardiomyocytes, the bulbus arteriosus (arrowheads in B and C), and in epicardial cells (arrows in C). (D) Ventral view of an adult heart. Expression of sox9b:EGFP in cardiomyocytes continues into adulthood. V is ventricle; At, atrium; BA, bulbus arteriosus. Anterior is to the left in all panels. Scale bars, 50 microns.