Figure 1.

Regulation of cellular contractility for apical constriction. (A) Epithelial invagination, as found during ventral furrow formation in the gastrulating Drosophila embryo, is a morphogenetic movement where apical constriction of epithelial cells that are joined to one another by cell–cell adhesion leads to inward folding of the epithelial sheet. At the apical poles of cells, actomyosin is found adjacent to the adherens junctions (zonula adherens) and in a medioapical network at the apical cortex. (B) Ratchet-like apical constriction. Apical constriction occurs in a step-wise fashion, driven by cycles of contraction and relaxation in the medioapical actomyosin network. The apical poles do not relax fully, constituting a ratchet for each cycle of contraction. (C) Hierarchical control of medioapical actomyosin via cycling of GTP- and GDP-loaded RhoA through the action of activator (RhoGEF2) and inactivator (C-GAP).