Figure 1. Stress relaxation in cell monolayers involves a change in length.

(a) Schematic diagram of the stress relaxation experiments. Monolayers were stretched to 30% strain at a 75%.s^-1 strain rate using a motorised micromanipulator and then kept at a fixed strain for ~130-140 s. The flexible rod was then returned to its initial position and the monolayers were left to recover. (b) Bright-field microscopy images of an epithelial monolayer before and during stretch. (Scale bar: 0.5 mm) (c) Stress relaxation curves of cell monolayers (n=17). The magenta and green dashed lines show 0 s and 30 s after application of stretch. Stresses go to zero upon return of the flexible rod to its initial position (t=140 s, black dashed line). (d) Bright-field microscopy images of Drosophila larval wing discs before and during stretch. (Scale bar: 100 μm) (e) Stress relaxation curves of Drosophila larval wing discs (n=12). (f) Confocal microscopy images of monolayers expressing E-Cadherin GFP for 0 s (left) and 30 s (middle) after stretch. Both images were overlayed to detect potential cell shape change during relaxation (right). (Scale bar: 10 μm) (g) Cross section of a monolayer expressing E-Cadherin GFP before application of stretch (−2.7 s), during stretch (0 s and 129.6 s) and upon release (136.4 s). The length of the monolayer upon release is different from its length before application of stretch. The monolayer appears in green, the surrounding medium appears in magenta due to inclusion of Alexa-647, and the glass substrate appears dark due to dye exclusion. The white dashed lines indicate the positions of the glass substrates. The part of monolayer situated between the two dashed lines is suspended. The dotted white line indicates the shape of the monolayer before application of stretch. (Scale bar: 100 μm). This experiment is representative of n=22 monolayers.