Many vertebrate organs are derived from monolayered epithelia that undergo morphogenesis to acquire their
shape. Whereas asymmetric left/right gene expression within the zebrafish heart field has been well documented, little
is known about the tissue movements and cellular changes underlying early cardiac morphogenesis. Here, we
demonstrate that asymmetric involution of the myocardium of the right-posterior heart field generates the ventral floor,
whereas the noninvoluting left heart field gives rise to the dorsal roof of the primary heart tube. During heart tube
formation, asymmetric left/right gene expression within the myocardium correlates with asymmetric tissue morphogenesis.
Disruption of left/right gene expression causes randomized myocardial tissue involution. Time-lapse analysis
combined with genetic analyses reveals that motility of the myocardial epithelium is a tissue migration process. Our
results demonstrate that asymmetric morphogenetic movements of the 2 bilateral myocardial cell populations generate
different dorsoventral regions of the zebrafish heart tube. Failure to generate a heart tube does not affect the acquisition
of atrial versus ventricular cardiac cell shapes. Therefore, establishment of basic cardiac cell shapes precedes cardiac
function. Together, these results provide the framework for the integration of single cell behaviors during the formation
of the vertebrate primary heart tube.