Actin (white) accelerates toward a wound (bottom) over time (left to right)

Cells sustain rips and tears during processes such as migration. The plasma membrane is repaired within milliseconds, but resealing of the underlying actomyosin cortex can take several minutes. On page 785, Mandato and Bement investigate this process in frog oocytes and find that a polymerization zone and a contractile ring work together to close wounds.

Mandato and Bement find that the structure surrounding a wound is clearly contractile—square wounds round up, elongated wounds shorten fastest along their long axis, and rewounding causes nearby areas to spring open further. But contractility seems to be restricted to a narrow ring, whereas actin and myosin accumulate in a broader zone around this ring. Actin filaments can be tracked as they flow and accelerate into this zone, thus depleting the surrounding area. This process of cortical flow should create a positive feedback loop, as incoming actin increases contractility, thus increasing flow.

Flow is probably not, however, how things get started. In the absence of actin, several proteins implicated in actin polymerization accumulate around a wound. De novo polymerization triggered by these proteins would normally provide the initial signal that sets up a zone of actin and myosin accumulation. Mandato and Bement show that the zone can be established in the absence of any flow, although the zone is unstable without contraction and does not move to heal the wound. Presumably the zone normally triggers enough contraction to initiate flow, and the flow then results in the formation of a contractile ring and healing. â–ª