Fibronectin regulates epithelial organization during myocardial migration in zebrafish.

Several genes have been implicated in heart tube formation, yet we know little about underlying cellular mechanisms. We analyzed the cellular architecture of the migrating myocardial precursors, and find that they form coherent epithelia that mature as they move medially. Mutant analyses indicate that the cardia bifida locus natter (nat) is required for the integrity of the myocardial epithelia. We positionally cloned nat and show that it encodes Fibronectin. During myocardial migration, Fibronectin is deposited at the midline between the endoderm and endocardial precursors, and laterally around the myocardial precursors. Further analyses show that Fibronectin deposition at the midline is required for the timely migration of myocardial precursors, but dispensable for the migration process itself. In the complete absence of Fibronectin, adherens junctions between myocardial precursors do not form properly, suggesting that cell-substratum interactions are required for epithelial organization. These data suggest that myocardial migration is dependent on epithelial integrity.