The effect of fibroblasts, vascular smooth muscle cells, and pericytes on sprout formation of endothelial cells in a fibrin gel angiogenesis system

We have recently shown that during angiogenesis in situ, sprouting and newly formed capillaries appear to be composed of two cell types, endothelial cells and nonendothelial, pericyte-like cells. The effect of pericytes on the process of neovessel formation is largely unknown. To study the influence of nonendothelial cell types on endothelial tubule formation, we have performed coculture experiments in a fibrin-clot angiogenesis system. When seeded below a critical density on the surface of fibrin gels, endothelial cells (from macro- or microvascular origin) did not show spontaneous formation of sprouts. However, in superconfluent cell cultures or after stimulation of endothelial cells with basic fibroblast growth factor (bFGF), endothelial cells frequently acquired an elongated shape. By stimulation of endothelial cells with both bFGF and vascular endothelial growth factor (VEGF), development of short capillary-like structures was induced. When endothelial cells were cocultivated with a cell type of high fibrinolytic potential, i.e., fibroblasts, development of capillary-like formations could not be detected. Cocultivation of endothelial cells with vascular smooth muscle cells or with retinal pericytes also did not increase the number of capillary-like formations in fibrin gels. In contrast, vascular smooth muscle cells on their own could be demonstrated to give rise to branched capillary-like networks in fibrin, which easily could be mistaken for true capillaries. Our results indicate that periendothelial cells contribute to angiogenesis not only by fibrinolysis and proteolytic permeation of the extracellular matrix. Rather, the interactions of endothelial cells and pericyte-like cells, as frequently observed during neovessel formation in situ, appear to be more specific and may require factors hitherto unknown.