During angiogenesis, new blood vessels sprout from an existing vascular network, elongate and bifurcate to form a new branching network. The individual and collective movements of vascular endothelial cells (ECs) during angiogenic morphogenesis are poorly understood but, on , Koichi Nishiyama and colleagues provide some new insights into these movements. Using time-lapse imaging and a computer-assisted analysis system to quantitatively characterise EC behaviours during sprouting angiogenesis, they show that ECs move backwards and forwards at different velocities and change their positions relative to each other, even at the tips of elongating branches in vitro. This ‘cell mixing’, which also occurs in vivo at the tips of developing mouse retinal vessels, is counter-regulated by EC-EC interplay via Dll4-Notch signalling and might be promoted via EC-mural cell interplay. Finally, the researchers show, the dynamic behaviour and migration of ECs contribute to effective branch elongation. Thus, cell behaviours during angiogenesis and other forms of branching morphogenesis might be more complex and variable than previously thought.
