(a) Prospective mesoderm cells on the ventral surface of the Drosophila embryo constrict their apical surfaces. This generates a bend in the tissue that causes the cells to invaginate to form a ventral furrow (dark gray). These cell shape changes are associated with an apical actin-myosin network (red). Before (top) and during (bottom) furrow formation. Lateral views, anterior left, ventral down (left), cross-sections (right). (b) Apical actin-myosin networks (red) also drive apical constriction of amnioserosa cells (dark gray), which generates one force that pulls the lateral epidermis closed over the dorsal surface of the Drosophila embryo. Contraction of the leading edge cable (thick red line), amnioserosa cell death, and filopodial protrusions also contribute to dorsal closure. (c) A medial actin-myosin network that spans the apical cell surface (light red) is connected through a second, junctional population that is anchored to adherens junctions at cell-cell contacts (dark red). (d) Recent studies demonstrate that apical constriction occurs in brief pulses associated with fluctuations in the actin-myosin network. Apical constriction is closely correlated with bursts of myosin accumulation.