The complexity of early heart development is reflected in the heterogeneity of progenitor cells and their distinct specification and differentiation mechanisms. The earliest segregation of progenitors is into the first and second heart fields, and while derivatives from these populations are found within all four heart chambers, the mechanism of atrial/ventricular (A/V) specification remains an open question. Now, Nicole Dubois, Robert Sebra, Kristin Beaumont and colleagues present their single-cell transcriptomic atlas of early cardiac development, which they interrogate to establish lineage relationships and to identify intermediate populations and their specification mechanisms. Deploying cell clustering and RNA velocity analysis, they confirm that the differentiated state depends on heart field progenitor identity but find that these analyses do not capture the segregation of the A/V lineages. Instead, the authors use lineage tracing to identify dynamically regulated genes that are likely to have a role in early A/V segregation. Finally, they dissect how aberrant expression of retinoic acid, a causative agent of heart defects, affects early progenitor populations and find that it causes a block in ventricular differentiation but not in early progenitor specification. Together, these data provide a rich resource for understanding heart development and identify some key developmental trajectories in the specification of chamber-specific cardiomyocytes.
