Figure 4.

Shear-strain events guide multicellular flow patterns. (a) First time frame of one side of a MDCK expansion assay. The monolayer edge moving along the x axis is divided into 33 sectors of 13 μm length. Scale bar = 100 μm. (b) Schematic construction of a protruding cells kymograph. (Left) Monolayer state at times 0–3. (Right) Stepwise recording of protrusion events in a kymograph (see Movie S2 for a step-by-step reconstruction). (c) Protruding cells kymograph. (Color encodes the position of the recorded protrusion event along the x axis.) (d) Detection of shear-strain events (black dots) in the protruding cells kymograph. (e) Snapshot of a frame taken 4 h after scratching. Velocity fields (red arrows) seem to converge onto shear-strain events (green asterisks). Scale bar = 100 μm. (f) Flow probability kymograph calculation. For each time point, tracers (blue circles) are placed randomly inside the monolayer and iteratively (labeled by i = 0–4) displaced along the snapshot of the motion field at that time point (red vectors). The final accumulation of tracers along the monolayer edge defines the flow probabilities (right). Note that this process repeats for each time-frame, freezing the corresponding velocity fields to repeatedly displace the tracers. (g) Flow probability kymograph. (Color encodes the probability of a tracer to reach the corresponding sector along the monolayer edge.) (h and i) Time-accumulated shear-strain events (h) and flow probabilities (i), p < 0.003 via permutation test (see Materials and Methods), Pearson R = 0.83, mean scrambled Pearson R = 0.73. (j) Shear-strain events guide long-distance multicellular dynamics.