(a) A single non-muscle myosin II motor translocates toward the plus end of an actin filament (left). However, it has a low duty ratio and thus spends only a small fraction of its time bound to the actin filament. Because of this, the motor is non-processive and does not move continuously along the actin filament for long distances. Gray arrow indicates the direction of motor movement. (b) Several myosin motors can assemble into a processive, bipolar filament that generates relative movement between two anti-parallel actin filaments. Gray arrows indicate the direction of actin filament movement. (c) A contractile network formed from many actin filaments and bipolar myosin filaments. Myosin motor activity causes the network to contract.

(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.

(a) The germband epithelium (dark gray) lengthens and narrows to elongate the body axis. Before (left) and after (right) elongation. Lateral views, anterior left, ventral down. (b) Junctional myosin (red) is localized to vertical interfaces between anterior and posterior cells, including single cell interfaces and multicellular cables. Laser ablation experiments reveal that myosin-rich interfaces are under tension, with the highest tension in multicellular cables. (c) Polarized cell rearrangements contribute to elongation of the body axis. Contraction of a single myosin-rich interface promotes local neighbor exchange (top), and the coordinated contraction of several adjacent cell interfaces forms a multicellular rosette structure that promotes many-cell rearrangements (bottom). Mechanical tension promotes multicellular cable formation, recruiting myosin to the cortex in regions under high tension.