Bleb (cell biology)
In cell biology, a bleb is a bulge or protrusion of the plasma membrane of a cell, human bioparticulate or abscess with an internal environment similar to that of a simple cell, characterized by a spherical, bulky morphology. It is characterized by the decoupling of the cytoskeleton from the plasma membrane, degrading the internal structure of the cell, allowing the flexibility required to allow the cell to separate into individual bulges or pockets of the intercellular matrix. Most commonly, blebs are seen in apoptosis (programmed cell death) but are also seen in other non-apoptotic functions. Blebbing, or zeiosis, is the formation of blebs.
Bleb growth is driven by intracellular pressure generated in the cytoplasm when the actin cortex undergoes actomyosin contractions. The disruption of the membrane-actin cortex interactions are dependent on the activity of myosin-ATPase
Bleb formation can be initiated in two ways: 1) through local rupture of the cortex or 2) through local detachment of the cortex from the plasma membrane. This generates a weak spot through which the cytoplasm flows, leading to the expansion of the bulge of membrane by increasing the surface area through tearing of the membrane from the cortex, during which time, actin levels decrease. The cytoplasmic flow is driven by hydrostatic pressure inside the cell.
Blebbing is one of the defined features of apoptosis. During apoptosis (programmed cell death), the cell's cytoskeleton breaks up and causes the membrane to bulge outward. These bulges may separate from the cell, taking a portion of cytoplasm with them, to become known as apoptotic blebs.Phagocytic cells eventually consume these fragments and the components are recycled.
Two types of blebs are recognized in apoptosis. Initially, small surface blebs are formed. During later stages, larger so-called dynamic blebs may appear, which may carry larger organelle fragments such as larger parts of the fragmented apoptotic cell nucleus.
Blebbing also has important functions in other cellular processes, including cell locomotion, cell division, and physical or chemical stresses. Blebs have been seen in cultured cells in certain stages of the cell cycle. These blebs are used for cell locomotion in embryogenesis. The types of blebs vary greatly, including variations in bleb growth rates, size, contents, and actin content. It also plays an important role in all five varieties of necrosis, a generally detrimental process. However, cell organelles do not spread into necrotic blebs.
In 2004, a chemical known as blebbistatin was shown to inhibit the formation of blebs. This agent was discovered in a screen for small molecule inhibitors of nonmuscle myosin IIA and was shown to lower the affinity of myosin with actin, thus altering the contractile forces that impinge on the cytoskeleton-membrane interface.
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