A tightly stretched sheet lies on a table for days. Outwardly it's still, but the fabric fibers retain tension — loosen one edge slightly, and wrinkles will run across it, rearranging the whole pattern. That's roughly how a layer of epithelial cells behaves at rest. It doesn't just freeze; it accumulates mechanical stress — a kind of cellular memory of past movements.
When serum is added to the medium (a wake-up signal), these pre-stressed regions become sources of collective behavior. From them, like little stars, waves radiate out, setting the direction of migration. At the junctions of such waves, topological defects arise — points where cell orientation changes abruptly, much like vortices in a crowd. These defects don't just drift; they collide and annihilate, and they do so far faster than if they were driven only by elastic forces.
The active elastic model built by the authors precisely reproduced this dynamics. It turned out that the mechanical memory of a resting epithelium is not a passive trace but a trigger mechanism. Upon waking, it releases stored energy, driving the cell sheet to reorganize into collective motion through the swift ordering of defects. This resembles neither the classic unjamming of a dense packing nor a transition to a swarming regime: it's a distinct, mechanically programmed route to coordinated migration.
An epithelial sheet that appears utterly motionless can store mechanical tension for years — much like a drawn bowstring remembers the archer's pull.
In Peter Watts's novel Echopraxia, wounded soldiers use "smart" bandages that guide cell migration, dictating the healing pattern through mechanical signals.