The number of cardiomyocytes in embryonic hearts was determined using single confocal sections (1 μm thick) of ventricles that were captured using a Zeiss LSM 710 confocal microscope with a 25× objective. The number of Mef2+DAPI+ nuclei in ventricles stained with anti-MHC was manually quantified using ImageJ software. The number of cardiomyocytes was determined by calculating the average number of Mef2+DAPI+ nuclei in three sections from each heart.