PCB126 induces a ventricular-specific reduction in the number of myocytes. (A–F) Ventral views of whole-mount Tg( cmlc2 ::DSRed-nuc) transgenic fish, which express red fluorescent protein (RFP) in cardiomyocyte nuclei. Ventricle viewed through the pericardial sac, cranial to the right. (A–C) show normal growth of the ventricle from 48 to 96 hpf in controls. (D–F) show the reduction in size of the PCB-exposed heart over the same time period. The cells of the ventricle become small and clustered (compare panels C and F). Hearts of representative fish were excised, and the cells of the atrium and ventricle were counted. (G) Myocyte counts at 48 hpf. There is a small difference in the number of both atrial and ventricular myocytes at this stage, although the difference in ventricular myocytes is not significant at p = 0.05. (H) Myocyte counts at 72 hpf. There is no significant difference in the number of atrial myocytes between control and PCB126-exposed fish, but there is a 35% reduction in the number of ventricular myocytes in the PCB126-exposed fish at this stage ( p = 0.01). Data were analyzed using two-tailed student's t -test. Bars show mean ± SE. N = 6 for each group.