Mouse heart samples were fixed in 4% paraformaldehyde for 3.5 hours, rinsed in PBS for 30 min, and immersed in PBS containing 30% sucrose overnight at 4°C. Heart samples were embedded in OCT for cryosectioning. Cryosections were stained with a chicken anti-GFP primary antibody (Abcam, ab13970) and a goat anti-chicken Alexa Fluor 488 (Life Technologies, A-11039) as previously described [1 (link)], followed by imaging. For whole-mount imaging, the left ventricles of uninjured hearts and infarcts were carefully removed and trimmed before imaging. Imaging was performed using a Leica SP8 confocal microscope or a Nikon A1 confocal microscope. Images obtained from cryosections were used for cell size measurement using ImageJ. Only EGFP+ cells with Dapi signals were included in cell size measurement. In case that cell boundaries between adjacent EGFP+ cells could not be identified, the entire EGFP+ area divided by the number of EGFP+ cells in this area was used as the average EGFP+ cell size in this area. Three samples per group were included in the analysis. At least three random images were used to calculate the average EGFP+ cell size of each sample.