(A) Representative immunofluorescence images of CF proliferation. LysMcre and mKO mouse hearts were prelabeled with 100 mg/kg EdU 24 hours and 9 hours before harvesting on post-MI day 3. Cryosections of the heart were assessed by Click iT kit to detect Edu (pink) and PDGFRα (green). Nuclei are stained with Hoechst (blue). Scale bars: 50 μm. Arrows show proliferating myofibroblasts, i.e., PDGFRα⁺ and EdU⁺ cells. (B) Quantification of proliferating myofibroblasts as a percentage of all PDGFRα⁺ cells in the infarcted region (n = 4–5/group). Dots represent biological replicates, and data represent the mean ± SEM. ***P < 0.001, by unpaired, 2-tailed Student’s t test. (C) Immunofluorescence imaged of fibronectin in infarcted heart tissue at post-MI day 7. Scale bars: 50 μm. (D) Quantitative morphometry of immunostaining, in which the relative abundance of the stained area was calculated by averaging the results from multiple independent images from f/f and mKO mice (n = 4/group). **P < 0.01, by unpaired, 2-tailed Student’s t test. (E) Representative immunofluorescence images of α-SMA staining in the infarcted region at post-MI day 7: fibronectin (red), α-SMA (green), Hoechst (blue). Scale bars: 50 μm. (F) Quantification of α-SMA⁺ staining in heart tissue at post-MI day 7 (n = 4 per group). *P < 0.05, by unpaired, 2-tailed Student’s t test. (G and H) mRNA expression levels of col1α (G) and α-SMA (H) were determined by qPCR in CFs treated with supernatants of spent medium of cM cells or control medium. cM cells were isolated from infarcted heart tissue at post-MI day 3 and then cultured in DMEM with 10% FBS for 2 hours or 24 hours. The supernatants of the spent medium or control medium were collected and added to the CF culture for an additional 24 hours. RNA was extracted from CFs and then subjected to qPCR (n = 4/group). Dots represent biological replicates, and data represent the mean ± SEM. **P < 0.01 and ***P < 0.001, by 1-way ANOVA.