Live-imaging reveals Coordinated Cell Migration and Cardiac Fate Determination during Mammalian Gastrulation

Heart development involves the specification of cardiac progenitors at distinct stages and locations. Using live-imaging of mouse embryos between gastrulation and heart tube formation, we tracked individual mesodermal cells and reconstructed their lineage trees for up to five cell divisions. We found independent unipotent progenitors emerging at specific times, contributing exclusively to either left ventricle/atrioventricular canal (LV/AVC) or atrial myocytes. LV/AVC progenitors differentiated early to form the cardiac crescent, while atrial progenitors later generated the heart tube’s inflow tract during morphogenesis. We also identified short-lived bipotent progenitors with broad potential, illustrating early developmental plasticity. Sister cells from bipotent progenitors displayed greater dispersion and more diverse migratory trajectories within the anterior mesoderm than those from unipotent progenitors. Bipotent progenitors contributing to extraembryonic mesoderm (ExEm) exhibited the fastest and most dispersed migrations, whereas those giving rise to endocardial, LV/AVC, and pericardial cells showed a more gradual divergence, with late-stage behavioural shifts: endocardial cells increased in speed, while pericardial cells slowed relative to LV/AVC cells. Together the data reveal the regulation of individual cell directionality and cardiac fate allocation within the seemingly unorganised migratory pattern of mesoderm cells.