| Summary: | We hypothesized that the neonatal rat heart would bring transplanted human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) to maturity as it grows to adult size. In neonatal rat heart, engrafted hiPSC derivatives developed partially matured myofibrils after 3 months, with increasing cell size and sarcomere length. There was no difference between grafts from hiPSC-CMs or hiPSC-derived cardiac progenitors (hiPSC-CPs) at 3 months, nor was maturation influenced by infarction. Interestingly, the infarcted adult heart induced greater human cardiomyocyte hypertrophy and induction of cardiac troponin I expression than the neonatal heart. Although human cardiomyocytes at all time points were significantly smaller than the host rat cardiomyocytes, transplanted neonatal rat cardiomyocytes reached adult size and structure by 3 months. Thus, the adult rat heart induces faster maturation than the neonatal heart, and human cardiomyocytes mature more slowly than rat cardiomyocytes. The slower maturation of human cardiomyocytes could be related to environmental mismatch or cell-autonomous factors.
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