Cell blebbing occurs when the outer membrane forms spherical, balloon-like bulges that protrude from the main body of the cell. These protrusions, called blebs, result from the flexible plasma membrane temporarily bulging outwards. The formation and retraction of blebs is a dynamic process, with a bleb growing and shrinking over a period of about two minutes.
The Process of Bleb Formation
Bleb formation is a mechanical process that begins when a small patch of the cell’s plasma membrane detaches from the underlying protein scaffold, the actin cytoskeleton. This internal framework provides structural support and shape to the cell. A rupture or weakening in this actin cortex creates a vulnerable spot on the cell’s surface.
Once this detachment occurs, the process is driven by the cell’s internal pressure. The cytoplasm is under constant hydrostatic pressure, which forces it toward the point of weakness, pushing the unsupported membrane outwards. This rapid influx of cytosol causes the spherical bulge to inflate and expand, forming a mature bleb in as little as 30 seconds.
The final stage involves the stabilization and retraction of the bleb. Shortly after the bleb expands, the cell reassembles the actin cytoskeleton just inside the bleb’s membrane. Actin-binding proteins attach to the inner surface, followed by the polymerization of new actin filaments. Motor proteins like myosin II are then recruited, forming a new contractile cortex that pulls the bleb back toward the cell body.
Biological Roles of Cell Blebbing
Blebbing is a feature of several biological processes, most notably programmed cell death, or apoptosis. During apoptosis, a cell initiates a self-destruct sequence to prevent its contents from damaging neighboring cells. Blebbing is a characteristic of this process, as the dying cell’s surface erupts with numerous blebs. These protrusions eventually pinch off, breaking the cell into smaller, membrane-enclosed fragments called apoptotic bodies that are consumed by immune cells.
The process is also observed during cell division, specifically in the final step known as cytokinesis. As one cell prepares to divide into two, blebbing helps facilitate the complex shape changes required for the physical separation of the new daughter cells. This contributes to the forces that ultimately cleave the parent cell into two distinct entities.
Some cells use blebbing as a method of movement, especially when navigating complex, three-dimensional environments like body tissues. A cell can generate a bleb at its leading edge, shifting its internal contents forward. The subsequent retraction of the bleb then helps to pull the rest of the cell body along, allowing it to squeeze through tight spaces.
Connection to Disease and Cellular Stress
The mechanical process of blebbing can be co-opted in disease states, particularly in cancer. Aggressive cancer cells can use blebbing to invade surrounding tissues and enter blood or lymphatic vessels. This amoeboid-like movement allows tumor cells to navigate through the dense matrix of tissues, contributing to metastasis, the spread of cancer to distant sites.
Blebbing also serves as a visible indicator of a cell under stress or injury. Exposure to toxins, extreme heat, or a lack of oxygen can damage the actin cytoskeleton, leading to the membrane detaching and triggering bleb formation. This can be a sign of irreversible injury leading to a chaotic form of cell death known as necrosis. Unlike the orderly process of apoptosis, necrosis involves the cell swelling and bursting, releasing its contents and causing inflammation.