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Although the mammalian heart's ability to fully regenerate is debated, its potential to extensively repair itself is gaining support. We hypothesized that heart regeneration relies on rapid angiogenesis to support myocardial regrowth and sought to characterize the timeline for angiogenesis and cell proliferation in regeneration.

One-day-old CD-1 mice (P1, N = 60) underwent apical resection or sham surgery. Hearts were explanted at serial time points from 0 to 30 days postresection and analyzed with immunohistochemistry to visualize vessel ingrowth and cardiomyocyte migration into the resected region. Proliferating cells were labeled with 5-ethynyl-2′-deoxyuridine injections 12 hours before explant. 5-Ethynyl-2′-deoxyuridine–positive cells were counted in both the apex and remote areas of the heart. Masson's trichrome was used to assess fibrosis.

By 30 days postresection, hearts …