Phenotypic overlap between hematopoietic cells with suggested angioblastic potential and vascular endothelial cells

The existence of angioblast-like circulating endothelial progenitor cells (EPC) in adult humans has been suggested recently. Their role in postnatal angiogenesis is under intensive investigation. Discrimination between the supposed angioblasts (AC133(+)/FLK-1(+)/CD34(+)) and mature endothelial cells (ECs) is complicated by the fact that subsets of hematopoietic cells express markers similar to those of ECs. Among these, monocytes/macrophages and monocyte-derived dendritic cells (DCs) are more differentiated hematopoietic cell populations. They show a wide phenotypic overlap with particularly sinusoidal and microvascular ECs. Furthermore, under local angiogenic growth conditions, monocytes or monocyte precursors or immature DCs may differentiate into endothelial-like cells (ELC). Initial evidence suggests an endothelium-independent revascularization potential carried by macrophages. These macrophages have been shown to form "tunnel-like structures" in ischemic regions. Future studies will need to address the question of whether monocyte-/dendritic cell-derived ELC can develop a similar functional behavior in vasoregulation, coagulation, and fibrinolysis, as described for vascular ECs, and thus may contribute to neoangiogenesis by a direct vessel-forming role.