The bHLH transcription factor Hand2 plays parallel roles in zebrafish heart and pectoral fin development

The precursors of several organs reside within the lateral plate mesoderm o f vertebrate embryos. Here, we demonstrate that the zebrafish hands off loc us is essential for the development of two structures derived from the late ral plate mesoderm - the heart and the pectoral fin. hands off mutant embry os have defects in myocardial development from an early stage: they produce a reduced number of myocardial precursors, and the myocardial tissue that does form is improperly patterned and fails to maintain tbx5 expression. A similar array of defects is observed in the differentiation of the pectoral fin mesenchyme: small fin buds form in a delayed fashion, anteroposterior patterning of the fin mesenchyme is absent and tbx5 expression is poorly ma intained, Defects in these mesodermal structures are preceded by the aberra nt morphogenesis of both the cardiogenic and forelimb-forming regions of th e lateral plate mesoderm, Molecular analysis of two hands off alleles indic ates that the hands off locus encodes the bHLH transcription factor Hand2, which is expressed in the lateral plate mesoderm starting at the completion of gastrulation. Thus, these studies reveal early functions for Hand2 in s everal cellular processes and highlight a genetic parallel between heart an d forelimb development.