Venous and arterial identity is predetermined in the embryo, with embryonic vessels expressing Eph-B4 differentiating into veins and vessels expressing ephrin-B2 differentiating into arteries. The specialized membrane organelles lipid rafts and caveolae serve as localized domains for proteins to interact with one another and play a role in signal transduction and vesicular trafficking. Several tyrosine kinase membrane receptors, including Eph-B1, have been colocalized to caveolae. These data suggest that caveolae and Eph receptors may have coordinated roles in determining vessel identity, not only during embryogenesis but perhaps also during adult vascular remodeling and angiogenesis.