30.3: Types of Membrane Protrusions
The protrusion of the cell surface is an initial step for several cellular processes, including cell migration, phagocytosis, and neurite outgrowth. These membrane protrusions are a result of cytoskeletal rearrangement. The most widely observed cell protrusions include lamellipodia, pseudopodia, filopodia, microvilli, invadopodia, and podosomes. These protrusions can be of two types — static or dynamic.
The microvilli, an example of stable protrusions, are finger-like projections with a generally stable actin architecture. These brush-like extensions are found on the apical surface of epithelial cells lining internal organs, such as the intestine. They greatly increase the surface area for enhanced nutrient absorption.
Contrastingly, dynamic protrusions are primarily involved in cell migration. The majority of these structures use actin polymerization as the driving force for membrane deformation. Additionally, actin binds myosin motor proteins to form contractile bundles. The myosin motors pull on the actin filaments, thus rearranging them during migration. Based on their appearance, actin-dependent protrusions are classified as lamellipodia, pseudopodia, filopodia, and invadopodia.
Many migrating cells like keratinocytes or fibroblasts exhibit lamellipodia, which are thin, sheet-like projections at the leading edge of these cells. They are involved in the rapid migration of cells to sites of injury for repair and regeneration. Lamellipodia can extend bundled actin filaments to form finger-like projections called filopodia. For example, neurons form these outgrowths to sense environmental signals and rewire the nervous system after trauma.
Other membrane protrusions include the podosomes and invadopodia. While podosomes are transient peg-like structures developed by migrating cells like macrophages, invadopodia are projections produced by metastatic cells on the extracellular matrix. Additionally, invadopodia secrete proteolytic enzymes, degrading the extracellular matrix to break tissue architecture and metastasize into adjacent tissues. Lastly, blebs are spherical membrane projections formed due to internal hydrostatic pressure. They do not depend on the actin cytoskeleton for their enlargement. Fibroblasts and immune cells exhibit bleb formation for migration and other cellular processes such as apoptosis.
