Arteries, Veins, Notch, and VEGF

Arteries and veins are the two most fundamental bloodvessel types: Higher-pressure, oxygenated blood flowsoutward through arteries, and lower-pressure, deoxygenated blood returns via veins. The existence of thesetwo intertwined yet distinct vascular networks has beenappreciated for thousands of years. The difference between arteries and veins was noted by the Greekanatomists Praxagoras and Herophilus in the third century B. C. (Wiltse and Pait 1998). In the 17th century,William Harvey established the functional definition ofarteries and veins as vessels that carry blood away fromthe heart and toward the heart in a continuous circulation(Harvey 1970). Since Harvey's time, there has been agreat deal of additional description of arteries and veinsand the morphological differences that distinguish them.These morphological differences include the presence ofa thicker vascular smooth muscle-containing wall aroundarterial vessels, and the presence of valves and other specialized structures found primarily within venous vessels(Fig. 1). The origins of the morphological differences between arteries and veins have generally been attributed tophysiological factors such as blood flow and hemodynamic pressure (Clark 1918; Gonzales-Crussi 1971; Girard 1973), and it has been assumed that the endothelialcells lining both types of vessels are essentially naivewith respect to their arterial or venous identity, at leastinitially. In recent years, however, information has begunto be forthcoming about the molecular aspects of bloodvessel determination challenging prevailing views on thenature and origins of arteries and veins...