<i>Notch1</i> is pan-endothelial before vascular remodeling and only limited to the arteries after vascular remodeling.

<p>Expression of <i>Notch1</i> was studied by <i>in situ</i> hybridization on mouse embryos at 5 somites (A), 8 somites (B), 12 somites (C) and E9.5 (D, n = 6 per stage). <i>Notch1</i> expression is absent in arteries at 5 somites (red arrow). When blood flow initiates at 8 somites, <i>Notch1</i> expression is found both in arteries (red arrow) and veins (blue arrow) at 8 somites and at 12 somites. Venous expression of <i>Notch1</i> is absent after vascular remodeling at E9.5. To investigate if flow is required for the expression of Notch1, we stained for <i>Notch1</i> expression in embryos in which had ablated flow. <i>Notch1</i> expression is present in the dorsal aorta of the control embryos (E, n = 4) but no expression is observed in embryos with ablated flow (F, n = 5). Scale bar; 50μm (A-C), 100 μm (D-F).