gridlock, an HLH gene required for assembly of the aorta in zebrafish

The first artery and vein of the vertebrate embryo assemble in the trunk by migration and coalescence of angioblasts to form endothelial tubes. The gr idlock (grl) mutation in zebrafish selectively perturbs assembly of the art ery (the aorta). Here it is shown that grl encodes a basic helix-loop-helix (bHLH) protein belonging to the Hairy/Enhancer of the split family of bHLH proteins. The grl gene is expressed in Lateral plate mesoderm before vesse l formation, and thereafter in the aorta and not in the vein. These results suggest that the arterial endothelial identity is established even before the onset of blood flow and implicate the grl gene in assignment of vessel- specific cell fate.