Mechanical signatures of structural disorder in the Drosophila germband epithelium*

Epithelial cells collectively generate functional tissues in developing embryos. In some morphogenetic events, a tissue dramatically reorganizes its internal structure - often generating high degrees of structural disorder - to accomplish changes in tissue shape. However, the origins of structural disorder in epithelia and what roles it might play in morphogenesis are not understood. We study this question in the Drosophila germband epithelium, which undergoes dramatic changes in internal structure as cell rearrangements drive elongation of the embryo body axis. Using two order parameters that quantify volumetric and shear disorder, we show that structural disorder increases during body axis elongation and is strongly linked with specific developmental processes. Both disorder metrics begin to increase around the onset of axis elongation, but then plateau at values that are maintained throughout the process. Notably, the disorder plateau values are similar to those for random cell packings, suggesting this may reflect a limit on tissue behavior. We also studied mutant embryos with disrupted internal and / or external stresses and find that the disorder metrics are delayed in time or reduced in magnitude compared to in wild-type embryos. Together, these findings demonstrate that both internal cell rearrangements and external stresses contribute to epithelial tissue disorder and suggest that the values of disorder in a developing tissue might reflect physical or biological limits on morphogenesis.