Bridges that guide and unite

摘要

Many primitive organisms lack blood vessels, and oxygen simply diffuses to their innermost cells. Blood vessels arose in evolution when organisms outgrew the physical limits of oxygen diffusion. They are lined with endothelial cells and branch by sprouting - sending out specialized endothelial cells called tip cells at the forefront of the sprout that navigate to their target1. Previous studies have mainly focused on understanding how tip cells initiate vessel sprouting; so nearly nothing is known about how these cells fuse with neighbouring sprouts to form a perfused vessel. Writing in Blood, Fantin et al. report that, contrary to expectation, tip cells lack the precision to find, recognize and fuse with other tip cells. Instead, macrophages, phagocytic cells of the immune system, serve both as what we would like to call 'bridge cells' and as guidance posts to precisely position tip cells in preparation for accurate fusion.