Figure 1. Mechanisms of arterial and venous determination during development of the embryonic vasculature.(A) In arterial endothelial cells, Sonic Hedgehog induces vascular endothelial growth factor (VEGF), which in turn activates Delta-Notch signaling. This cascade induces expression of the arterial-specific marker Ephrin-B2, and simultaneously limits expression of the venous-specific marker Eph-B4.(B) In venous endothelial cells, COUPTFII actively blocks expression of the VEGF receptor, and subsequently expression of the Delta-Notch cascade. The failure of this cascade prevents expression of Ephrin-B2 while simultaneously stimulates Eph-B4 expression.