The cellular mechanics of cephalic furrow formation in the Drosophila embryo investigated using an advanced vertex model

Cephalic furrow formation (CFF) is one of the major morphogenetic movements in the Drosophila (fruit fly) embryo. The furrow initially develops at the lateral sides of the embryo and then extends over the entire embryo perimeter. An analysis of the cell shape changes that occur in the active region of the epithelial cell layer reveals that the CFF occurs in two distinct phases: the initiation and progression phases. In the initiation phase, the epithelial fold starts to invaginate into the yolk sac and during the progression phase, additional pairs of cells are gradually added to the furrow. While the geometry of the cell shape changes during these processes was imaged in detail, the underlying mechanical forces and stresses

are poorly understood. To investigate the invagination mechanics, we have developed an advanced vertex model that takes into account experimentally observed membrane curvature and relates it to a pressure jump across the membrane. Our model shows that the cell shape sequences observed in the progression phase can be fully explained by a corresponding succession of cell membrane length changes and the corresponding time variation of the membrane tension and cell pressure. However, the initiation of the CFF requires an additional inward force associated with the tension that develops along the curved line of the furrow cleft.