The mechanics of cephalic furrow formation in the Drosophila embryo

Abstract

Cephalic furrow formation (CFF) is a major morphogenetic movement during gastrulation in Drosophila melanogaster embryos that gives rise to a deep, transitory epithelial invagination. Recent studies have identified the individual cell shape changes that drive the initiation and progression phases of CFF; however, the underlying mechanics of these changes are not yet well understood. During the progression phase, the furrow deepens as columnar cells from both the anterior and posterior directions fold inwards rotating by 90{degrees}. To analyze the mechanics of this process, we have developed an advanced 2D vertex model, which introduces multi-node representation of cellular membranes and allows us to capture the membrane curvature associated with pressure variation. Our investigations reveal some key mechanical features of CFF. As cells begin to roll over the cephalic furrow cleft, they become wedge-shaped as their apical cortices and overlying membranes expand, lateral cortices and overlying membranes release tension, internal pressures drop, and basal cortices and membranes contract. Cells then reverse the process by shortening apical cortices and membranes, increasing lateral tension, and causing internal pressures to rise. Since the basal membranes expand, the cells recover a rotated columnar shape at the end of this process. Interestingly, our findings indicate that the basal membranes may be passively reactive throughout the progression phase. We also find that the smooth rolling of cells over the cephalic furrow cleft necessitates that internalized cells provide a solid base through high membrane tensions and internal pressure levels, which allows transmission of tensile force that pulls new cells into the furrow. These results lead us to suggest that CFF may help establish a baseline tension across the apical surface of the embryo that would facilitate cellular coordination of other morphogenetic movements via mechanical stress feedback mechanisms. SIGNIFICANCEMechanical forces and stress feedback are essential for the development of morphology and structure in the embryo. Although great progress has been made in understanding the genetic control of patterning and cell fate, mechanical stress contributions are not as well understood. Mechanical analyses of the apical constrictions initiating ventral furrow formation and subsequent invagination dynamics in Drosophila have shed considerable light on these processes; however, ventral furrow formation is only one of many morphogenetic movements. Cephalic furrow formation occurs simultaneously with ventral furrow formation, but its cell shape changes and invagination dynamics are radically different. This study shows that mechanical forces and feedback operating in cephalic furrow formation also differ considerably from those in ventral furrow, demonstrating a potentially wide array of mechanical processes in morphogenesis.