Spontaneous migration of cellular aggregates: from giant keratocytes to running spheroids

We introduce the broad field of entangled active matter. Unlike swarms of fish and flocks of birds, cells are bound by transient links and behave as active viscoelastic pastes.
Here, we investigate the collective migration of cell on adhesive gels, using 3D cellular aggregates as a model system. Aggregates spread by expanding outwards a cell monolayer, which may partially dewet, causing the aggregate to move. Varying the substrate rigidity induces different modes of aggregate motion: “Giant Keratocytes”, where the lamellipodium is a cell monolayer that expands at the front and retracts at the back; “Penguins”, characterized by bipedal locomotion; and “Running Spheroids”, for non-spreading aggregates. We characterize these diverse modes of collective migration by quantifying the flows and force field responsible of the bipedal stick-slip motion.
We propose two possible mechanisms to explain the spontaneous migration of cellular aggregates: i)chemical modification of the substrate in analogy to reactive droplets. We show that it is possible to mimic a keratocyte with a droplet of oil containing a surfactant. The reactive droplet adopts a croissant shape also seen for keratocyte fragments and ii) symmetry-breaking arising from cell polarization in analogy to active droplets.