Quantitative Aspects of Interface Remodeling During Germband Extension

Oriented cell intercalation is an essential developmental process that shapes tissue morphologies through the directional insertion of cells between their neighbors. Intercalary behaviors in the early Drosophila embryo occur through a remodeling of cell topologies, with cells contracting shared AP interfaces to a single point, followed by newly juxtaposed DV cells constructing horizontally-oriented interfaces between them. Previous research has focused on properties of cell-cell interfaces, and led to a model in which actomyosin networks mediate higher line tensions at AP interfaces to direct contraction. However, the contribution of tricellular vertices to tissue elongation remains unclear. This study shows that cell intercalation uses a novel sliding vertex mechanism that physically couples vertices to radially-oriented forces. Through live imaging and quantitative analysis it was observed that the motion of vertices at contracting interfaces is not coupled, but instead vertices demonstrate strong radial coupling across the area of cells. The vertices of AP junctions show independent sliding behaviors along the cell periphery to produce the topological deformations responsible for intercalation. AP junctions undergo ratcheted length changes that are coordinated with cell area oscillations. These results suggest a model in which oscillations in cell area direct the progressive, ratcheted motion of individual vertices to drive oriented cell intercalation and tissue extension in the Drosophila epithelium. In a second study, analysis of germband extension in 4D revealed that interface contraction and T2 formation can initiate from any point along on the apical-basal