Mechanical coupling narrows an epithelial colony before it reaches a bottleneck
A colony migrating into a passage narrower than itself but wider than individuals must adapt its density distribution to pass. Powders, colloids and crowds driven through such an opening jam, since contact interactions provide no pathway for constituents to respond to the constriction before reaching it. Exploring whether a cohesive living tissue can reorganize in advance, we image MDCK epithelial monolayers migrating through microchannels in which a wide channel narrows, and locate response at the taper mouth, a transition zone in which we identify upstream responses. At locations where the local channel width is still unchanged, the monolayer detaches from the side walls and reduces its lateral extent. Particle image velocimetry shows that this is not passive compression; the axial velocity alternates aperiodically in sign, velocity divergence is positive in the transition zone, and tracking the positions of cell nuclei confirms that cells leave the crowded inlet region in both directions. Fluorescence imaging shows a belt of co-enriched E-cadherin, F-actin, and myosin-II at the edges that detach before the inlet, which laser ablation shows to be under elevated tension but dissolves once the tissue passes into the narrow inlet; {beta}-catenin is not co-enriched, indicating that junctions differ in composition in the core and at the edge of colony. This advance proceeds over hours, far more slowly than the viscoelastic reorganization reported for MDCK monolayers, and switches in sign repeatedly, which a linear viscoelastic sheet under sustained constraint cannot do. E-cadherin knockdown abolishes narrowing, flow reversal, density redistribution, and belt assembly.