Statement: Tracing the morphogenesis and transcriptome of early vascular smooth 33 muscle cells using foxc1b 34 35 Running title: Early smooth muscle cell development and transcriptome 36 37 Keywords: foxc1b, acta2, vascular smooth muscle cells, zebrafish, development, RNA-Seq 38 transcriptome 39 40 Abstract: 41Despite the critical role of vascular mural cells (smooth muscle cells and pericytes) in supporting 42 the endothelium of blood vessels, we know little of their early morphogenesis and differentiation. 43 foxc1b:EGFP expressing cells in zebrafish associate with the vascular endothelium (kdrl) and 44 co-express a smooth muscle marker (acta2), but not a pericyte marker (pdgfrβ). The expression 45 of foxc1b in early peri-endothelial mesenchymal cells allows us to follow the morphogenesis of 46 mesenchyme into acta2 expressing vascular smooth muscle cells. We show that mural cells 47