Phenotypic overlap between monocytes and vascular endothelial cells.

During embryonic development, endothelial cells (ECs) develop organ specific properties. ECs express specific markers, which are helpful in identifying these cells in vivo and in culture. Interestingly, most of the supposed specific endothelial markers are present on both ECs and hematopoietic precursors or mature blood cells, which correspond to the idea of a common embryonic precursor. Monocytes/makrophages and monocyte-derived dendritic cells, as more differentiated hematopoietic cell populations, show a wide phenotypic overlap with particularly hepatic sinusoidal, and microvascular endothelial cells within inflamed tissue, such as neovascularizised complicated atherosclerotic plaques. Furthermore, under local angiogenic growth conditions monocytes or monocyte precursors or immature dendritic cells may differentiate into endothelial like cells. First evidence suggests an endothelium-independent revascularization potential carried by monocyte-derived macrophages. These macrophages have been shown to form tunnel-like structures in ischemic regions. Future studies have to address the question, whether monocyte-/dendritic cell-derived endothelial like cells can develop a similar functional behaviour in vasoregulation, coagulation and fibrinolysis, as described for vascular endothelial cells, and thus may contribute to neoangiogenesis by a direct vessel-forming role.