Design and fabrication of a working tissue model of the developing embryonic heart

Introduction:: Morphogenesis is a major developmental process that forms the proper structures in utero through a series of highly coordinated structural deformations. Despite the importance, there have been limited models of morphogenesis, outside of simple cellular or animal models, due to the complexity of the hierarchical structural change. In this study, we combined tissue engineering with developmental biology in an approach called engineered development to build a tissue model of embryonic heart morphogenesis. In development, the embryonic heart undergoes a complex morphogenesis, known as looping, transforming the primordial tube to adult four-chambered organ. An in vitro model of morphogenesis could provide unique insights into this crucial phase of development and improve understanding of congenital defect generation.

Materials and Methods:: Embryonic heart tubes (EHT) were engineered using a novel casting method through serial castings resulting in an inner acellular layer surrounded by a thin iPS derived cardiomyocyte layer (Fig 1a), mimicking the in vivo anatomy. In the developing heart, looping is driven through both internal and external forces, leading to elongation of the tube resulting in a rightward rotation at the cranial end of the embryo and a dorsal deflection at the caudal end. We visualized and simplified (Fig 1b) the external forces exhibited upon the heart tube in the embryo. These simplified forces were then translated into an externally stimulating bioreactor (Fig 1c). The bioreactor is controllable and dynamic and allows for control on both poles of the heart tube to mimic the forces on those ends. EHTs are able to be loaded into the bioreactor and perfusion cultured directly within the bioreactor.

Results, Conclusions, and Discussions:: EHTs were able to through the looping process (Fig 1d) by recapitulating the major structural milestones during the looping process, culminating in a fully looped four-chambered structure. Fully looped tubes were perfusion cultured for seven days (Fig 1e) and were found to have fused together into a "mini-heart". Upon histological investigation, "mini-hearts" were found to have structures matching native heart tissue. Histological examination discovered intraventricular septum formation, cellular orientations matching measured angles within mammalian hearts, and ventricular shapes resembling native heart (Fig 1f). In conclusion, through engineered development, a model of cardiogenesis was created. This model could be further expanded to further study cardiogenesis or modified to study the onset of congenital heart defects. Additionally, in the future we envision translating the concept of engineered development to other organ types to generate three-dimensional embryogenesis models.

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