Elongating human heart organoids recapitulate early cardiac morphogenesis and axial organization.

Early heart development involves heart tube elongation, looping, and axial patterning, yet these processes remain difficult to study experimentally. While pluripotent stem cell-derived heart organoids model cardiomyocyte differentiation, they do not recapitulate early morphogenetic events. Here, we generate elongating heart organoids (EHOs) from human induced pluripotent stem cells that undergo coordinated elongation and looping-like curvature, resembling early cardiac morphogenesis. EHOs establish a venous-to-arterial axis with spatially organized sinus venosus-like, atrial, and ventricular cardiomyocytes, and exhibit sequential propagation of contractile activity along this axis. Single-cell transcriptomics and trajectory analyses, together with pulse-labeling, support a model in which progressive incorporation of cardiac cells from proliferative splanchnic mesodermal cells at the venous pole drives elongation of the cardiac structure. Consistent with in vivo phenotypes, TBX5 deletion results in shortened EHOs with reduced looping-like curvature and irregular contractions. Together, EHOs provide a human in vitro system that enables investigation of early cardiac morphogenesis.