A systematic analysis of neonatal mouse heart regeneration after apical resection

Abstract

The finding that neonatal mice are able to regenerate myocardium after apical resection has recently been questioned. We determined if heart regeneration is influenced by the size of cardiac resection and whether surgical retraction of the ventricular apex results in an increase in cardiomyocyte cell cycle activity. We performed moderate or large apical ventricular resections on neonatal mice and quantified scar infiltration into the left ventricular wall at 21 days post-surgery. Moderately resected hearts had 15±2% of the wall infiltrated by a collagen scar; significantly greater scar infiltration (23±4%) was observed in hearts with large resections. Resected hearts had higher levels of cardiomyocyte cell cycle activity relative to sham hearts. Surgically retracting the ventricle often resulted in fibrosis and induced cardiomyocyte cell cycle activity that were comparable to that of resected hearts. We conclude that apical resection in neonatal mice induces cardiomyocyte cell cycle activity and neomyogenesis, although scarring can occur. Surgical technique and definition of approach to assessing the extent of regeneration are both critical when using the neonatal mouse apical resection model.

Keywords: Apical resection; Cardiac regeneration; Fibrosis; Neomyogenesis.

Figure 1. Regeneration and scarring occur after apical resection

Hearts were isolated at 3 hours post-resection, 21 dpr and 7 dpr. A. Relative heart weights at 3 hours post-resection. (Sham: n=21, Moderate: n=25, Large: n=9). *p<0.05,***p<0.001. B. Representative trichrome stainings of sections from hearts at 21 dpr. Apical regions of the left ventricle are shown. Scar tissue stains blue. Scale bar: 100 µm. C. Quantification of scar incursion into the left ventricular wall (prop.=proportion). (Sham: n=6, Moderate: n=23, Large: n=10). *p<0.05. See also, Supporting Figure 2. D. Representative immunofluorescent stainings of sections at 7 dpr for phospho-H3 (pH3) and cardiac troponin t (cTnnT). Arrowheads indicate pH3+/cTnnT+ cells. E. Quantification of pH3+/cTnnT+ cells. (Sham: n=6, Moderate: n=8, Large: n=8). *p<0.05,**p<0.01, ns=non-significance.

Figure 2. Surgical retraction results in fibrosis and an increase in cardiomyocyte cell cycle activity

Hearts were isolated and stained at 7 dpr. A. Image depicting the surgical retraction procedure. B. Representative images of non-retracted and retracted sham hearts. C. Trichrome staining of sections from non-retracted and retracted sham hearts. Apical regions of the left ventricle are shown. Scale bar: 100 µm. D. Representative stainings of sections at 7 dpr for phospho-H3 and cardiac troponin t. Arrowheads indicate pH3+/cTnnT+ cells. E. Quantification of pH3+/cTnnT+ cardiomyocytes. (Non-retracted Sham: n=6, Retracted Sham: n=10). **p<0.01.