What guides early embryonic blood vessel formation?

Survival of vertebrate embryos depends on their ability to assemble a corre ctly patterned, integrated network: of blood vessels to supply oxygen and n utrients to developing tissues. The arrangement of larger caliber intraembr yonic vessels, specification of arterial-venous identity, and proper placem ent of major branch points and arterial-venous connections are all precisel y determined, A number of recent studies in both mammalian and nonmammalian vertebrate species, reviewed here, have now begun to reveal the major role played by genetically predetermined extrinsic cues in guiding the formatio n of early embryonic blood vessels and determining the global pattern of th e vasculature. (C) 1999 Wiley-Liss,Inc.