Mechanical Inference Reveals the Germband Epithelium as a Non-Conventional Material*

Embryo and organ development depend on mechanical forces that shape and regulate cell and tissue structures. Measuring force is thus essential for analyzing the mechanisms that control tissue morphogenesis, and recent non-invasive methods based on cell shape and connectivity offer promising insights. In this study, we apply Bayesian inference to the converging and extending Drosophila germband epithelium, which exhibits planar-polarized myosin II and experiences anisotropic forces. We found that edges oriented along the dorsal-ventral (DV) axis exhibit higher tensions and shorter lengths compared to the longer, lower-tension edges along the anterior-posterior (AP) axis. These tension results are consistent with the observed higher myosin intensities at vertical cell edges compared to horizontal ones and with prior laser ablation measurements. This confirms a clear correlation between junctional myosin II intensity and edge tension. Interestingly, the tissue-level stress-strain curve shows an initial elastic buildup followed by plastic flow or failure, while the stress-strain rate plot displays highly non-Newtonian behavior, characterized by shear stiffening followed by shear thinning.