Wang, H.; Andersen, P.; Inoue, T.; Hibino, N.; Lee, D. I.; Kwon, C.

Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) offer tremendous potential for cardiac disease modeling and regenerative therapies, but their clinical application is limited by their immature characteristics. Here we show that in vivo bioincubation in neonatal rat hearts enables hiPSC-CMs to achieve structural maturity, exhibiting features of adult cardiomyocytes, including organized sarcomeres, rod-shaped morphology, and multinucleation. We further provided proof-of-concept evidence for engraftment in neonatal pig hearts for maturation, supporting feasibility in large animal models. The use of large-particle cell sorting enables recovery of intact, adult-sized cardiomyocytes for subsequent analysis. These findings establish a practical and scalable platform for generating structurally mature human cardiomyocytes through in vivo bioincubation.