Spontaneous rotations in epithelia as an interplay between cell polarity and boundaries

Directional flows of cells have been observed during development in a variety of systems ranging from Drosophila to zebrafish. These flows shape living matter in phenomena involving cell mechanics and regulation of the acto-myosin cytoskeleton and are important for morphogenesis. However, the onset of the observed coherent motion is still poorly understood. Here we identify the inherent coherence length to show that coherence is associated with spontaneous alignments of cell polarity. We use cellular rings of controlled dimensions and live tracking of cellular shapes and motions under various experimental conditions, finding that a tug-of-war between cell polarities within the ring dictates the onset of coherence. In addition, we identify an internally driven constraint set by cellular acto-myosin cables at the inner and outer ring boundaries. As these structures have a high RhoA protein activity, they confine the cells and are essential to ensure coherence. The finding that acto-myosin cables are required to trigger coherence is supported by numerical simulations based on a Vicsek-type model that includes free active boundaries. We quantitatively reproduce coherence onsets. We propose that spontaneous coherent motion results from basic competitions between cell orientations and active cables at boundaries.