The lamellipodium (: lamellipodia) (from

Latin Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...

''lamella'', related to ', "thin sheet", and the Greek radical ''pod-'', "foot") is a

cytoskeletal The cytoskeleton is a complex, dynamic network of interlinking protein filaments present in the cytoplasm of all Cell (biology), cells, including those of bacteria and archaea. In eukaryotes, it extends from the cell nucleus to the cell membrane ...

protein actin projection on the leading edge of the cell. It contains a quasi-two-dimensional actin mesh; the whole structure propels the cell across a substrate. Within the lamellipodia are ribs of actin called microspikes, which, when they spread beyond the lamellipodium frontier, are called filopodia. The lamellipodium is born of actin

nucleation In thermodynamics, nucleation is the first step in the formation of either a new Phase (matter), thermodynamic phase or Crystal structure, structure via self-assembly or self-organization within a substance or mixture. Nucleation is typically def ...

in the plasma membrane of the cell and is the primary area of actin incorporation or

microfilament Microfilaments, also called actin filaments, are protein filaments in the cytoplasm of eukaryotic cells that form part of the cytoskeleton. They are primarily composed of polymers of actin, but are modified by and interact with numerous other ...

formation of the cell.

Description

Lamellipodia are found primarily in all mobile cells, such as the

keratinocytes Keratinocytes are the primary type of cell found in the epidermis, the outermost layer of the skin. In humans, they constitute 90% of epidermal skin cells. Basal cells in the basal layer (''stratum basale'') of the skin are sometimes refer ...

of fish and frogs, which are involved in the quick repair of wounds. The lamellipodia of these keratinocytes allow them to move at speeds of 10–20 μm / min over epithelial surfaces. When separated from the main part of a cell, a lamellipodium can still crawl about freely on its own. Lamellipodia are a characteristic feature at the front, leading edge, of motile cells. They are believed to be the actual motor which pulls the cell forward during the process of

cell migration Cell migration is a central process in the development and maintenance of multicellular organisms. Tissue formation during embryogenesis, embryonic development, wound healing and immune system, immune responses all require the orchestrated movemen ...

. The tip of the lamellipodium is the site where exocytosis occurs in migrating mammalian cells as part of their clathrin-mediated endocytic cycle. This, together with actin-polymerisation there, helps extend the lamella forward and thus advance the cell's front. It thus acts as a steering device for cells in the process of

chemotaxis Chemotaxis (from ''chemical substance, chemo-'' + ''taxis'') is the movement of an organism or entity in response to a chemical stimulus. Somatic cells, bacteria, and other single-cell organism, single-cell or multicellular organisms direct thei ...

. It is also the site from which particles or aggregates attached to the cell surface migrate in a process known as cap formation. ;Structure Structurally, the barbed ends of the microfilaments (localized actin

monomer A monomer ( ; ''mono-'', "one" + '' -mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or two- or three-dimensional network in a process called polymerization. Classification Chemis ...

s in an ATP-bound form) face the "seeking" edge of the cell, while the pointed ends (localized actin monomers in an ADP-bound form) face the lamella behind. This creates treadmilling throughout the lamellipodium, which aids in the retrograde flow of particles throughout. Arp2/3 complexes are present at microfilament-microfilament junctions in lamellipodia, and help create the actin meshwork. Arp2/3 can only join onto previously existing microfilaments, but once bound it creates a site for the extension of ''new'' microfilaments, which creates branching. Another molecule that is often found in polymerizing actin with Arp2/3 is cortactin, which appears to link tyrosine kinase signalling to cytoskeletal reorganization in the lamellipodium and its associated structures. Rac and Cdc42 are two

Rho Rho (; uppercase Ρ, lowercase ρ or ; or ) is the seventeenth letter of the Greek alphabet. In the system of Greek numerals it has a value of 100. It is derived from Phoenician alphabet, Phoenician letter resh . Its uppercase form uses the same ...

-family

GTPase GTPases are a large family of hydrolase enzymes that bind to the nucleotide guanosine triphosphate (GTP) and hydrolyze it to guanosine diphosphate (GDP). The GTP binding and hydrolysis takes place in the highly conserved P-loop "G domain", a ...

s which are normally cytosolic but can also be found in the cell membrane under certain conditions. When Cdc42 is activated, it can interact with Wiskott–Aldrich syndrome protein (WASp) family receptors, in particular N-WASp, which then activates Arp2/3. This stimulates actin branching and increases cell

motility Motility is the ability of an organism to move independently using metabolism, metabolic energy. This biological concept encompasses movement at various levels, from whole organisms to cells and subcellular components. Motility is observed in ...

. Rac1 induces cortactin to localize to the cell membrane, where it simultaneously binds F-actin and Arp2/3. The result is a structural reorganization of the lamellipodium and ensuing cell motility. Rac promotes lamellipodia while cdc42 promotes filopodia. Ena/VASP proteins are found at the leading edge of lamellipodia, where they promote actin polymerization necessary for lamellipodial protrusion and chemotaxis. Further, Ena/VASP prevents the action of capping protein, which halts actin polymerization.

References

External links

MBInfo - Lamellipodia

MBInfo - Lamellipodia Assembly

Video tour of cell motility
* {{Authority control Cell movement Cytoskeleton Actin-based structures de:Lamellipodium