Cardiomegaly of exercised hearts occurs via hyperplasia, as indicated by an increased number of cardiomyocytes per cross-sectional area. There were 8% more CM nuclei per measured cardiomyocyte (CSA) in EX ventricular sections than Control. (A) CSA was measured as a 0.5 mm²-box overlaid on standardized dimension digital micrographs. The digital micrograph is a representative Control heart section. Images were created by overlaying the fluorescent image of the section with the DAPI channel (blue, for all nuclei) on top of the image obtained with the TRITC channel (red, for cardiomyocytes). (B) There were 5.82 CM nuclei/CSA in EX hearts, versus 5.37 in Control hearts. N=9 hearts for EX and 8 hearts for Control. For each heart, CM nuclei counts were performed on sections from two different tissue depth levels, and then averaged together. Error bars represent SEM. Statistical significance: *p<0.05.