The embryonic heart tube forms from bilateral groups of cardiomyocytes that move towards the embryonic midline where they merge. The transcription factor Hand2 is essential for this ‘cardiac fusion’ but its downstream effectors are unknown. By studying zebrafish heart development, Deborah Yelon and colleagues now identify Fibronectin as a component of the Hand2 pathway that mediates cardiac morphogenesis (see p. ). By performing transplantation experiments between wild-type and hand2 mutant embryos, the researchers show that hand2 regulates cardiac fusion by altering the environment through which the cardiomyocytes migrate. Next, they show that fibronectin 1 (fn1) expression is increased in hand2 mutant embryos. Finally, they report that reduction of fn1 function rescues cardiac fusion in hand2 mutant embryos but not the apicobasal polarity defect that is also seen in these embryos. Thus, the Hand2 pathway regulates cardiac morphogenesis by establishing an appropriate environment for cardiac fusion by limiting Fibronectin function but it establishes the apicobasal polarity that is needed for heart tube extension through another, unidentified, effector.
