A bleb is a large, deformation on the surface of a cell which is almost spherical in shape. Blebs occur during apoptosis, to aid in cell deconstruction, cytokinesis, and cell movement. It has been observed that cell blebs begin and conclude in about 10 seconds and occur over about ten microns of cell surface.
During a bleb the cortical acto-myosin of the cell contracts. This generates a hydrostatic pressure causing a section of the plasma membrane to tear away from the cytoskeleton. This section which is lacking a cytoskeleton swells very quickly with cytosol, the intracellular fluid. As the swelling increases there is additional tearing of the plasma membrane along the cell increasing the size of the bleb. When swelling slows, a mesh of actin and myosin II assembles to form a contractile cortex which is attached to the plasma membrane and brings the bleb back toward the cell body.
Non-equilibration of hydrostatic pressure in blebbing cells, G. Charras, J. Yarrow, M. Horton, L. Mahadevan and T. Mitchison, Nature, 435, 365-69. 2005.