Figure 4.

Remodelling of epithelial sheets in Drosophila embryos. (A) Planar cell rearrangement is dependent on actomyosin assembly at the apical junctions. Polarized assembly of junctional actomyosin filaments is driven by tension. Actomyosin-rich junction(s) contract, which in some cases can lead to a transient ‘rosette'-like cell assembly that subsequently dissolves through the polarized expansion of new boundaries. Polarized junction contraction and expansion drives germ band elongation. During planar cell rearrangements, E-cadherin accumulated in ‘spot junctions', which are connected to stable actin patches and linked to each other through a dynamic actin network ‘slide' along the boundaries. (B) Epithelial invagination is driven by pulsed contractions of a non-junctional actomyosin network. During epithelial invagination, coalescing actomyosin networks form at the apex of the invaginating cells. These networks are connected with E-cadherin-based spot junctions. Stochastic centripetal contractions of the apical actomyosin networks, followed by the stabilization of the contracted states, drives apical constriction. E-cadherin-based spot junctions are essential for maintaining tissue integrity and stabilizing anisotropic cell shape changes during the contraction pulses.