The role of preexisting collateral channels in the heart is far less well established. Early studies failed to show the existence of coronary-to-coronary artery connection in normal human hearts7, 8; however, functional studies show that up to 25% of patients with coronary stenosis demonstrate the existence of significant collateral flow when challenged with a temporary coronary occlusion. 9 Finally, vasculogenesis is the process of an in situ formation of blood vessels from circulating endothelial progenitor cells (EPCs) and vascular progenitor cells. 10, 11 The functional significance of vasculogenesis in the setting of ischemia in coronary or peripheral circulation has not been established conclusively. Remarkably, the reported spectrum of benefit ranges from significant12 to none at all. 13 Clearly, this area requires much additional investigation. It should be noted that the choice of a model to study a particular neovascularization event may give undue prominence to one or the other processes mentioned above. Thus, in the case of tissue injury, angiogenesis is likely to be the predominant process, with little arteriogenesis taking place; however, in the case of a common femoral artery ligation, arteriogenesis will predominate at the site of ligation, whereas angiogenesis will predominate in the ischemic distal bed. Finally, in the case of a lethally irradiated mouse receiving a bone marrow transplant, vasculogenesis will predominate at the sites of injury.