Tissue macrophages: "satellite cells" for growing collateral vessels? A hypothesis.

It has been demonstrated in several studies that collateral growth is associated with accumulation of macrophages around proliferating vessel. Macrophages are known to secrete vascular growth factors and metalloproteinases. Both are necessary for the development of a proper vasculature. Recent studies suggest that certain subpopulations of macrophages are also capable of transdifferentiating into vascular cells. There are good reasons to assume that shear force rises dramatically in preexisting arteriolar shunts after occlusion of the main supplying vessel. Based upon these two findings it was hypothesized that high shear forces lead to homing of circulating monocytes to the growing collateral artery. The majority of studies, however, indicate that monocytes home under low shear force conditions. Our own observations in monocyte depleted animals suggest that proliferation and transdifferentiation of tissue macrophages occurs locally in growing collateral vessels and is independent of circulating cells. We thus propose that local proliferation and transdifferentiation of tissue macrophages rather than homing of circulating monocytes play a major role in arteriogenesis.