Blebs are a type of membrane protrusion formed by the internal hydrostatic pressure of the cytoplasm. Blebs are observed in several cell types, including fibroblasts, immune cells, and single-celled organisms like the amoeba. The primary function of blebs is cell locomotion and apoptosis, but they are also found during necrosis and cell division. The life cycle of a bleb comprises an initiation phase followed by the expansion and retraction phases.

Blebbing Through the Matrix

In multicellular organisms, migrating cells navigate through a complex three-dimensional path, squeezing their way between cells and through the extracellular matrix (ECM). The actin-driven membrane protrusions, such as filopodia and lamellipodia, aid in crawling across surfaces or between cells. In contrast, blebs allow the cells to navigate through narrow gaps between the components of the ECM. The initiation and expansion of blebs push the flexible plasma membrane through these small gaps while subsequent bleb retraction helps move the cell through this three-dimensional matrix.

Blebbing and Apoptosis

Apoptosis or programmed cell death is a sequential cascade of events that aid the death of a cell without damaging surrounding cells. Blebbing is a characteristic feature of this process. An apoptotic cell initiates multiple blebs on its surface, which pinch off to form tiny apoptotic blebs containing bits of cytoplasm and proteins from the primary cell. Other cells such as neutrophils and macrophages phagocytose these apoptotic blebs, and help recycle the cellular components. Apoptotic cells also produce larger blebs containing fragments of disrupted organelles such as mitochondria and nucleus.