Endothelial cells in 2-dimensional tissue culture are able to migrate (indicated by arrows), proliferate, and form chordlike structures. When placed in 3-dimensional culture in a collagen matrix, endothelial cells form tunnels and generate a lumen, thereby displaying properties that are considered essential for assembling the vasculature in vivo. Sacharidou et al identify a signaling complex, termed “endothelial lumen signaling complex,” in which the membrane-anchored metalloproteinase MT1-MMP interacts with the integrin α2β1, and both bind to the junctional adhesion molecule JamC, which in turn interacts with the related JamB. The cytoplasmic domains of JamB and JamC then bind directly to Par3 to recruit the intracellular components of the signaling complex, which also include Par6B and Cdc42. Professional illustration by Marie Dauenheimer.