GUIDED MIGRATION AS A NOVEL MECHANISM OF CAPILLARY NETWORK REMODELINGIS REGULATED BY BASIC FIBROBLAST GROWTH-FACTOR

To investigate mechanisms of capillary net work remodeling, we develop ed a serum-free angiogenesis in vitro system in three-dimensional fibr in matrices which allows the study of directional growth of endothelia l sprouts, anastomosis, and remodeling ('pruning') of the primitive pl exus toward more elaborated capillary trees. To follow the movements o f living endothelial cells by inverse-fluorescence microscopy, we cocu ltured unlabeled endothelial cells with endothelial cells labeled with the carbocyanine dye -dioctadecyl-3,3,3',3'-tetramethylindocarbocyani ne perchlorate (DiI). We show that elongation and retraction of neighb oring capillary sprouts occurs simultaneously, resembling a tug-of-war by which endothelial cells are withdrawn from shortening sprouts to b ecome incorporated in other sprouts nearby. For the first time, we dir ectly demonstrate the long-suspected parallel sliding movement of endo thelial cells. We show that cell migration persists within immature ca pillaries even after sprouts have merged to continuous capillary loops , leading to overlapping growth of opposing sprout tips. As a novel co ncept of capillary remodeling, we distinguish two types of endothelial cell migration: sprouting and guided migration. Sprouting is the de n ovo invasion of a matrix by endothelial cells, and guided migration is the locomotion of cells along preexistent capillary-like structures. We show that guided migration leads to remodeling of immature capillar y networks and to the retraction of sprouts. We describe a method for quantification of sprouting versus guided migration in DiI-mosaic-labe led capillary networks, and we present evidence that endothelial cell- derived basic fibroblast growth factor serves as a chemotactic signal for other cells to migrate along a preestablished capillary-like struc ture.