Bleb (cell biology)
In cell biology, a bleb is a bulge of the plasma membrane of a cell, characterized by a spherical, "blister-like", 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 for the cell to separate into individual bulges or pockets of the intercellular matrix. Most commonly, blebs are seen in apoptosis, but they are also seen in other non-apoptotic functions, including apocrine secretion. Blebbing, or zeiosis, is the formation of blebs.
Formation
Initiation and expansion
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 initiation is affected by three main factors: high intracellular pressure, decreased amounts of cortex-membrane linker proteins, and deterioration of the actin cortex. The integrity of the connection between the actin cortex and the membrane are dependent on how intact the cortex is and how many proteins link the two structures. When this integrity is compromised, the addition of pressure is able to make the membrane bulge out from the rest of the cell. The presence of only one or two of these factors is often not enough to drive bleb formation. Bleb formation has also been associated with increases in myosin contractility and local myosin activity increases.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. Before the bleb is able to expand, pressure must build enough to reach a threshold. This threshold is the amount of pressure needed to overcome the resistance of the plasma membrane to deformation.
Artificial induction
Bleb formation has been artificially induced in multiple lab cell models using different methods. By inserting a micropipette into a cell, the cell can be aspirated rapidly until destruction of cortex-membrane bonds causes blebbing. Breakage of cortex-membrane bonds has also been caused by laser ablation and injection of an actin depolymerizing drug, which in both cases eventually led to blebbing of the cell membrane. Artificially increased levels of myosin contractility were also shown to induce blebbing in cells. Some viruses, such as the poxvirus Vaccinia, have been shown to induce blebbing in cells as they bind to surface proteins. Although the exact mechanism is not yet fully understood, this process is crucial to endocytosing the virion and subsequent infection.Cellular function
Apoptotic function
Blebbing is one of the defined features of apoptosis. During apoptosis, 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.