Unveiling Complexity and Multipotentiality of Early Heart Fields

Rationale: Extraembryonic tissues, including the yolk sac and placenta, and the heart within the embryo, work to provide crucial nutrients to the embryo. The association of congenital heart defects with extraembryonic tissue defects further supports the potential developmental relationship between the heart and extraembryonic tissues. Although the development of early cardiac lineages has been well-studied, the developmental relationship between cardiac lineages, including epicardium, and extraembryonic mesoderm remains to be defined. Objective: To explore the developmental relationships between cardiac and extraembryonic lineages. Methods and Results: Through high-resolution single-cell and genetic lineage/clonal analyses, we show an unsuspected clonal relationship between extraembryonic mesoderm and cardiac lineages. Single-cell transcriptomics and trajectory analyses uncovered 2 mesodermal progenitor sources contributing to left ventricular cardiomyocytes, 1 embryonic and the other with an extraembryonic gene expression signature. Additional lineage-tracing studies revealed that the extraembryonic-related progenitors reside at the embryonic/extraembryonic interface in gastrulating embryos and produce distinct cell types forming the pericardium, septum transversum, epicardium, dorsolateral regions of the left ventricle and atrioventricular canal myocardium, and extraembryonic mesoderm. Clonal analyses demonstrated that these progenitors are multipotent, giving rise to not only cardiomyocytes and serosal mesothelial cell types but also, remarkably, extraembryonic mesoderm. Conclusions: Overall, our results reveal the location of previously unknown multipotent cardiovascular progenitors at the embryonic/extraembryonic interface and define the earliest embryonic origins of serosal mesothelial lineages, including the epicardium, which contributes fibroblasts and vascular support cells to the heart. The shared lineage relationship between embryonic cardiovascular lineages and extraembryonic mesoderm revealed by our studies underscores an underappreciated blurring of boundaries between embryonic and extraembryonic mesoderm. Our findings suggest unexpected underpinnings of the association between congenital heart disease and placental insufficiency anomalies and the potential utility of extraembryonic cells for generating cardiovascular cell types for heart repair.