Generation of Human Cardiac Organoids from Embryonic Stem Cells via Stepwise Mesoderm Induction and 3D Self-organization

Human embryonic stem cell (hESC)-derived cardiac organoids are multicellular three-dimensional (3D) structures that recapitulate key aspects of early human heart development and function. These self-organizing organoids exhibit spontaneous contractility, cardiomyocyte marker expression, and tissue-like architecture reminiscent of native myocardium. Here, we present a robust and reproducible protocol to generate cardiac organoids from the H9 hESC line via stepwise lineage differentiation. Mesoderm induction is initiated by treating spheroids for 36-40 h with Activin A (50 ng/mL), bone morphogenetic protein 4 (BMP4, 10 ng/mL), fibroblast growth factor 2 (FGF2, 30 ng/mL), laduviglusib (CHIR99021, 3 µM), and a phosphatidylinositol 3-kinase (PI3K) inhibitor (LY294002, 5 µM). Cardiac lineage specification is subsequently directed by daily exposure for four days to BMP4 (10 ng/mL), FGF2 (10 ng/mL), a Wnt pathway inhibitor (XAV-939, 5 µM), and retinoic acid (0.5 µM). Cardiomyocyte differentiation and maturation are further promoted from Day 5.5 onward using BMP4 (10 ng/mL), FGF2 (10 ng/mL), and insulin (10 µg/mL). Functional validation is achieved through time-lapse imaging and immunofluorescence analysis, confirming the generation of contractile cardiomyocytes marked by cardiac troponin T (cTnT) expression. Additionally, 3D immunostaining reveals the presence of α-SMA and CDH5, indicating the emergence of smooth muscle and endothelial-like cell populations. These cardiac organoids consistently demonstrate rhythmic contractions; however, direct electrophysiological validation of electromechanical coupling was not performed. Known limitations include the inability to passage organoids and potential central necrosis during extended cultures. In summary, this model provides a scalable and physiologically relevant platform for studying human cardiogenesis, drug responses, and congenital heart diseases.