Mechanical aspects of fruit fly gastrulation. Konstantin Doubrovinski^1,2, Bing He¹, Oleg Polyakov¹, Eric Wieschaus^1,2. 1) Princeton University, Princeton, NJ; 2) Howard Hughes Medical Institute.

   Epithelial morphogenesis plays a major role in embryonic development. During this process cells within epithelial sheets undergo complex spatial reorganization to form organs with specific shapes and functions. Fruit fly gastrulation serves a popular model of epithelial morphogenesis. In the course of gastrulation a subset of ventrally localized cells that constitute the mesodermal primordium constrict their apices thereby causing the tissue to bend into a crease termed ventral furrow. A number of signal transduction pathways regulating morphogenetic events that accompany gastrulation have been characterized in the past. However, physical mechanisms that underlie those morphogenetic events remain unclear. To tackle this problem we developed a novel particle velocimetry based approach for quantifying tissue deformation during the course of gastrulation. Our method involves injecting embryos with fluorescent tracer particles and tracking the motion of those particles over time. We demonstrate that the dynamics of deformation accompanying gastrulation is consistent with that of viscous flow. Specifically, we propose that surface deformation of the prospective mesoderm generated through apical constrictions causes a shearing force that brings about the motion of the cytoplasm in the interior of the tissue. We show that this simple physical description can quantitatively account for the measured velocity distribution acquired by particle tracking. In summary, our data suggests a physical mechanism through which apical constriction may translate into cell shape changes. Furthermore, our data suggests that the previously proposed physical mechanism of tissue invagination during fruit fly gastrulation may need to be revised.