Fig. 5: In theory and experiment, cell shape and collective dynamics discriminate UJT from pEMT.
The dynamic vertex model (DVM) attributes the effects of pEMT mainly to diminished edge tension but attributes those of UJT mainly to augmented cellular propulsion. a DVM predicts that during UJT versus pEMT two different metrics of cell shape diverge; aspect ratio (AR) emphasizes elongation whereas shape index q emphasizes perimeter (q = perimeter/(area1/2)). Increasing p0 (- - -) moderately increases AR but substantially increases q, resulting in somewhat elongated cells with tortuous edges. By contrast, increasing v0 (_._._) substantially increases AR but minimally increases q, resulting in elongated cells with straight edges. Measurements of AR and q from cells undergoing UJT (blue squares) or pEMT (red triangles) are consistent with those predictions. Insets show traced cell edges from representative images of cells in each state. During pEMT edge tension decreases as junctional adhesion decreases, and as cells elongate q increases more quickly than AR. Cell–cell junctions become increasingly tortuous and slack. During UJT, by contrast, edge tension increases as cellular propulsion v0 increases, AR and q increase in tandem, and cells elongate. Cell–cell junctions remain straight and taut. Theory and experimental data, taken together, suggest that layer fluidization by means of UJT versus pEMT follow divergent pathways. b DVM predicts that median pack size and average cell speed will, as persistence length l0 (…) increases, positively scale together. The time courses of median pack size and speed from jammed cells (black circles) or those undergoing pEMT (red triangles) did not follow this relationship. However, the time evolution of median pack size and speed from cells undergoing UJT (blue squares) are consistent with these predictions. During UJT, cellular cooperativity emerges and cells are able to move quickly in locally coordinated groups. Pack sizes, for both experimental and simulation data, were scaled by the size of the system, while cell speeds were scaled by the global maximum. Insets show representative dynamic packs from cells undergoing pEMT or UJT (scale bar: 100 µm). DVM simulation and experimental data, taken together, thus provide a physical picture of two distinct mechanisms of transition of a cellular collective from a solid-like to a fluid-like state.
