Decoding Fibroblast Diversity Associated with the Postnatal Loss of Cardiac Regenerative Capacity

The mammalian heart rapidly loses regenerative capacity after birth and responds to myocardial infarction (MI) with scar formation and development of interstitial fibrosis. Cardiac fibroblasts orchestrate extracellular matrix (ECM) remodeling and cell-cell communication during development and injury; however, how fibroblast heterogeneity and fibroblast communication networks differ between regenerative neonatal and non-regenerative adult hearts remains incompletely defined. We performed scRNA-seq analysis on metabolically active CD45^-/CD31^- nonmyocyte cells from the left ventricles of normal neonatal (P3) and adult (P84) mice to probe heterogeneity in a cardiac fibroblast-enriched population. We identified five transcriptionally distinct cardiac fibroblast subclusters (CF0-CF4) demonstrating different distributions across ages, including an adult-enriched immune/complement-associated program (CF0); an ECM structural-associated program present across ages (CF1); and neonatal-enriched contractile/ECM-remodeling (CF2), Wnt-modulating matrix-regulatory (CF3), and proliferative (CF4) programs. Matrisome category scoring revealed age-dependent divergence in ECM programs: neonatal fibroblasts showed higher enrichment of core matrisome components (particularly collagens and proteoglycans), whereas adult fibroblasts were relatively enriched for matrisome-associated categories, including ECM regulators and secreted factors. Ligand-receptor inference using CellChat demonstrated a broad reduction in fibroblast-fibroblast interaction strength and information flow in adult networks, and adult-enriched signaling was dominated by immune/chemotactic pathways. Finally, projection of subcluster marker programs onto an independent bulk RNA-seq dataset of cardiac fibroblasts 3 days after MI revealed that adult injury partially recapitulates neonatal-associated programs, including activation of the contractile/ECM-remodeling program (CF2) and robust induction of a cell-cycle-associated program (CF4), but lacks an additional neonatal-specific injury program associated with the Wnt-modulating subset (CF3), which was weakly induced or absent in adults. This cardiac fibroblast-enriched single-cell study defines age-dependent fibroblast states, ECM specialization, and communication network architecture that distinguish regenerative neonatal from non-regenerative adult hearts. It also provides a framework to interpret divergent stromal responses after MI and to prioritize fibroblast programs for regenerative and anti-fibrotic strategies.