Late in cortex reassembly, myosin (green) is recruited to foci at bleb membranes (red).

On page 477, blebbing cells open a window into the formation of a contractile actin cortex. Charras et al. use the blebs to order the events of cortex assembly.

Blebs form (usually during apoptosis or cytokinesis) as a result of strong actomyosin forces that separate a section of the plasma membrane from its underlying cortex and inflate it with cytosol. This free membrane is then a platform for new cortex assembly, which the authors watched by tracking potentially relevant proteins.

Cortex assembly occurred via three independent steps. First, ezrin and moesin—proteins that establish links between the membrane and cytoskeleton—were recruited to the blebs.

Actin polymerization and the membrane recruitment of several actin-binding proteins followed. The authors suspect that new actin is nucleated by a formin, based on filament morphology and the lack of Arp2/3 at the membrane, although the Dia1 formin was also missing. The relevant formin may be activated—either constitutively or in response to the loss of membrane–cortex contact—by RhoA, which had an unusually strong membrane localization in blebs.

The final step included the generation of force on the cortex, which followed myosin recruitment into foci at the membrane. That only a few foci exert force on the network suggests that cells might lock in tension by cross-linking actin filaments. The foci and the unpolarized actin network also distinguish the contractile cortex of blebbing cells from the ordered actomyosin network of muscle cells.