The novel in vitro and ex vivo model systems we have established, together with advanced techniques in gene editing and molecular analysis, allowed us to reach an unprecedented level of precision in modeling human cardiogenesis, from CPC specification to tissue-level morphogenesis and functional maturation processes. This was demonstrated by the novel insights gained by transitioning from 2D to 3D hiPSC-based modeling of HLHS, which provided the electromechanical environment needed to reveal the impaired maturation and cell cycle defects contributing to left ventricular hypoplasia, arguing against a solely hemodynamic origin of the defect (Krane et al. Circulation 2021). Another notable achievement was the generation of epicardioids, which are the first cardiac organoid recapitulating the formation of the epicardium, the outer layer of the heart (Meier et al. Nat. Biotechnol. 2023). Although the epicardium is quiescent in the healthy adult heart, it plays key roles during embryonic development and has been shown to orchestrate tissue repair in fetal mammals and in species capable of full heart regeneration throughout adulthood, such as zebrafish. Epicardioids therefore offer exciting new opportunities to study human epicardial development, which occurs too early in pregnancy to observe in native tissues, and to explore the epicardium’s potential for heart regeneration after an injury such as a myocardial infarction. Through lineage tracing in epicardioids, we could already demonstrate that human epicardial cells have the potential to give rise to cardiomyocytes, a long-standing open question in the field. We were also the first to identify the human equivalent of murine juxta-cardiac field (JCF) cells and to verify their dual potential for the myocytic and epicardial lineages, both in 2D and in epicardioids (Zawada et al. Nat. Commun 2023). Finally, although technical hurdles slowed down our progress in the generation of pig-human chimeras, we are confident that our characterization of porcine cardiogenesis and our efforts in generating cardiogenesis-disabled pig embryos not only served to advance cardiac modeling but also open new possibilities of testing interspecies cell complementation approaches ex vivo, which should facilitate the generation of chimeric tissues for xenotransplantation, an ambitious goal in regenerative medicine.
