Creating frog heart as an organ: in vitro-induced heart functions as a circulatory organ in vivo

Cardiomyocytes have been induced from various pluripotent cells, such as embryonic stem cells and myeloid stem cells; however, the generation of cardiac tissues beyond two-dimensional cell-sheets has not been reported. Creating higher order, three-dimensional structures that are unique to heart is the long-awaited next step in realizing cardiac regenerative medicine. We have previously shown that cardiomyocytes can be induced in vitro from undifferentiated cells (animal caps) excised from Xenopus embryos. Cardiomyocytes were induced by first dissociating the animal caps and then reaggregating them following treatment with activin. Here, we describe an interesting method for creating a complete ectopic heart in vivo, involving the introduction of in vitro-created tissue during early embryogenesis. Thus, animal cap reaggregates were transplanted into the abdomen of late-neurula-stage embryos, resulting in two-chambered hearts being formed. The dual-heart larvae matured into adult animals with transplanted hearts intact. Involvement of transplanted hearts in systemic circulation was demonstrated. Moreover, the ectopic hearts possessed higher order structures such as atrium and ventricle, and were morphologically, histologically, and electrophysiologically identical to original hearts. This system should facilitate the study of heart organogenesis and may promote a shift from tissue to organ engineering for clinical applications.