Movie 3.

Characterization of mechanical strain around the future head site in a regenerating tissue spheroid. Time-lapse, spinning-disk confocal movie of a regenerating tissue fragment expressing Lifeact-GFP in the ectoderm (Fig. 3A). The tissue spheroid was initially oriented with its future head region facing the objective, showing the focus region that develops into a +1 defect (orange labels). The images show computationally generated projected views of the fiber organization at the ectodermal basal surface (upper left), the cellular organization at the ectodermal apical surface (upper right), and cell segmentation maps depicting ectodermal cells colored according to their area (lower left; Range (white to red): 300-1200μm²) or cell anisotropy (lower right; Range (white to red): Q=0.1-0.65). The nematic orientation of the fibers in each cell is overlaid on the segmentation maps. Stretching events leading to mechanical strain focusing at the future head side are indicated by green frames, and are slowed down to better demonstrate the tissue deformations. The elapsed time from excision is displayed (hh:mm), and the scale bar is 100 μm.