Blebs are spherical cellular protrusions that occur in many physiological situations. Two distinct phases make up
the life of a bleb, each of which have their own biology and physics: expansion, which lasts ;30 s, and retraction, which lasts
;2 min. We investigate these phases using optical microscopy and simple theoretical concepts, seeking information on
blebbing itself, and on cytomechanics in general. We show that bleb nucleation depends on pressure, membrane-cortex
adhesion energy, and membrane tension, and test this experimentally. Bleb growth occurs through a combination of bulk flow of
lipids and delamination from the cell cortex via the formation and propagation of tears. In extreme cases, this can give rise to a
traveling wave around the cell periphery, known as ‘‘circus movement.’’ When growth stalls, an actin cortex reforms under the
bleb membrane, and retraction starts, driven by myosin-II. Using flicker spectroscopy, we find that retracting blebs are fivefold
more rigid than expanding blebs, an increase entirely explained by the properties of the newly formed cortical actin mesh.
Finally, using artificially nucleated blebs as pressure sensors, we show that cells rounded up in mitosis possess a substantial
intracellular pressure.