Breaking down symmetry

Actin machinery in vivo typically becomes polarized in response to external cues that trigger directional movement. In the absence of such signals, however, achieving polarization requires breaking part of the actin network. Such is the case for isolated cells at rest or for van der Gucht's experimental system, in which beads coated with actin polymerization proteins are mixed with actin monomers, ATP, and other proteins.During symmetry breaking, a fracture appeared at the outer actin rim, which then grew inward and expanded to open up a hole within the network. Once the hole was wide enough, the bead escaped...