Cellular blebbing in the ventral furrow. Jonathan S Coravos, Adam Martin. Massachusetts Institute of Technology, Cambridge, MA.

   Blebbing is a cellular process where the plasma membrane separates from the underlying actin cortex, inflates away from the point of separation, and finally retracts. Blebbing depends on actin cortex contraction by the molecular motor myosin II, which is thought to increase local hydrostatic pressure that causes blebs. While blebbing has been characterized in single cells, bleb function in a multicellular tissue has not yet been demonstrated. We investigate blebbing in the Drosophila ventral furrow (VF), a tissue invagination event at the onset of gastrulation. Because the VF blebs profusely, whereas neighboring epithelia do not, VF invagination is a promising place to identify roles for blebbing in multicellular tissues. Using 4-D confocal imaging with F-actin and membrane markers, we show that VF blebs have a similar cycle to those previously described in cultured cells. Similar to cell culture blebs, ventral furrow blebs exist for 30 to 60 seconds. Blebs coincide with local depletions in the actin and myosin cortex, expand as spherical actin-depleted bodies to about 3 m in diameter, and then accumulate actin during membrane retraction. Injecting embryos with the F-actin stabilizing drug, phalloidin, increases the density of blebs in the VF. Together with the observation that bleb retraction coincides with actin accumulation in the bleb, the phalloidin result supports a model in which F-actin turnover is required for cortex assembly in the bleb, which is also required for bleb retraction. Finally, we show that blebs occur with greater probability in the space between the cell centroid and cell-cell junctions, suggesting a heterogeneous organization of the cell cortex. We are investigating potential regulators of this organization, such as Moesin, an actin-membrane crosslinker.