Physical Aspects of <i>Drosophila</i> gastrulation.

Despite a long-standing effort to uncover the physical principles governing animal morphogenesis, the knowledge of embryonic tissue mechanics remains elusive. We address the physical aspects of embryonic tissue mechanics using fruit fly gastrulation as a model. During this process, a subset of cells in the ventral part of the embryo constrict on one side and subsequently invaginate into the interior of the embryo, thereby causing the embryonic surface to form a furrow. To determine the mechanism of tissue shape change during gastrulation, we have developed a toolbox of biophysical methods that allow accurate quantification of material tissue properties in live fruit fly embryos. Specifically, we have developed magnetic tweezers exploiting either fluorescent magnetic microspheres or ferrofluid droplets, as well as flexible cantilevers microfabricated from PDMS, allowing highly quantitative measurements of the rheological properties in the early fly embryo. Our measurements directly translate into a predictive theory that explains key aspects of tissue dynamics during fruit fly gastrulation. Specifically, our model explains the marked anisotropy of tissue constriction in the initial phase of gastrulation as well as the mechanism of tissue invagination during the subsequent phase.