Bone marrow (BM)-derived cells are thought to participate in the growth of blood vessels during postnatal vascular regeneration and tumor growth, a process previously attributed to stem and precursor cells differentiating to endothelial cells. We used multichannel laser scanning confocal microscopy of whole-mounted tissues to study angiogenesis in chimeric mice created by reconstituting C57BL mice with genetically marked syngeneic BM. We show that BM-derived endothelial cells do not significantly contribute to tumor-or cytokine-induced neoangiogenesis. Instead, BM-derived periendothelial vascular mural cells were persistently detected at sites of tumor-or vascular endothelial growth factor-induced angiogenesis. Subpopulations of these cells expressed the pericyte-specific NG2 proteoglycan, or the hematopoietic markers CD11b and CD45, but did not detectably express the smooth muscle markers smooth muscle ␣-actin or desmin. Thus, the major contribution of the BM to angiogenic processes is not endothelial, but may come from progenitors for periendothelial vascular mural and hematopoietic effector cells.

Endostatin, a fragment of collagen XVIII, is a potent antagonist of angiogenesis and inhibitor of tumor growth in mouse models. At present, the mechanism of action of endostatin is unknown. We show here that recombinantly produced human endostatin interacts with ␣5and ␣v-integrins on the surface of human endothelial cells. We further demonstrate that the endostatin-integrin interaction is of functional significance in vitro, as we found that immobilized endostatin supports endothelial cell survival and migration in an integrin-dependent manner. Soluble endostatin in turn inhibits integrin-dependent endothelial cell functions, such as cell migration. Taken together, these results implicate integrins as potential targets for endostatin function and support the importance of integrins in endothelial cell biology and angiogenesis.