Investigating Cell and Tissue Mechanics during Drosophila Embryogenesis using Laser Microsurgery

Abstract

Living tissues are active, non-linear viscoelastic materials that move drastically, often in concert, during embryogenesis. In many cases, the mechanics of this motion remain unknown. Using a combination of laser microsurgery and finite-element simulations, I explore the mechanics of Drosophila embryogenesis during two consecutive stages: germband retraction and dorsal closure. First, I investigate the interactions between two tissues, the amnioserosa and germband, as they move cohesively across the surface of the embryo during germband retraction. I find that the amnioserosa mechanically assists germband retraction but only by pulling on a few germband segments – specifically those around the curve. Retraction also depends on cell autonomous elongation in the germband, modeled by a polarization of cell edge tensions that aligns perpendicular rather than parallel to the principle stress direction. Cell elongation aligns with this polarization in most germband segments, but in a few, again mostly around the curve, cell elongation aligns with the direction of greatest anisotropic stress. Second, I probe the tension distribution within a single contracting tissue, the amnioserosa during dorsal closure. These tests demonstrate that the amnioserosa acts more like a continuous sheet than a cellular foam, where tensile stress is carried both by cell-cell interfaces and by an apical actin network. Together these results further our understanding of the physics of embryogenesis and provide a framework for future experiments probing how the mechanics change in mutants that fail to complete germband retraction or dorsal closure.