The lamellipodium (plural lamellipodia) (from

Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...

''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 cells, including those of bacteria and archaea. In eukaryotes, it extends from the cell nucleus to the cell membrane and is compo ...

protein actin projection on the leading edge of the

cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery ...

. 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 Filopodia (singular filopodium) are slender cytoplasmic projections that extend beyond the leading edge of lamellipodia in migrating cells. Within the lamellipodium, actin ribs are known as ''microspikes'', and when they extend beyond the lam ...

, which, when they spread beyond the lamellipodium frontier, are called

filopodia Filopodia (singular filopodium) are slender cytoplasmic projections that extend beyond the leading edge of lamellipodia in migrating cells. Within the lamellipodium, actin ribs are known as ''microspikes'', and when they extend beyond the lame ...

. The lamellipodium is born of actin nucleation 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 pr ...

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 referre ...

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 Epithelium or epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercell ...

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 embryonic development, wound healing and immune responses all require the orchestrated movement of cells in particular dire ...

. The tip of the lamellipodium is the site where exocytosis occurs in migrating mammalian cells as part of their

clathrin Clathrin is a protein that plays a major role in the formation of coated vesicles. Clathrin was first isolated and named by Barbara Pearse in 1976. It forms a triskelion shape composed of three clathrin heavy chains and three light chains. When ...

-mediated

endocytic cycle Endocytosis is a cellular process in which substances are brought into the cell. The material to be internalized is surrounded by an area of cell membrane, which then buds off inside the cell to form a vesicle containing the ingested material. ...

. 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. It is also the site from which particles or aggregates attached to the cell surface migrate in a process known as

cap formation When molecules on the surface of a motile eukaryotic cell are crosslinked, they are moved to one end of the cell to form a "cap". This phenomenon, the process of which is called cap formation, was discovered in 1971 on lymphocytes and is a propert ...

. ;Structure Structurally, the barbed ends of the microfilaments (localized actin

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

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 In molecular biology, treadmilling is a phenomenon observed within protein filaments of the cytoskeletons of many cells, especially in actin filaments and microtubules. It occurs when one end of a filament grows in length while the other end shri ...

throughout the lamellipodium, which aids in the retrograde flow of particles throughout.

Arp2/3 complex Arp2/3 complex (Actin Related Protein 2/3 complex) is a seven-subunit protein complex that plays a major role in the regulation of the actin cytoskeleton. It is a major component of the actin cytoskeleton and is found in most actin cytoskeleton ...

es are present at microfilament-microfilament junctions in lamellipodia, and help create the actin meshwork. Arp 2/3 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 Cortactin (from "''cortical actin'' binding protein") is a monomeric protein located in the cytoplasm of cells that can be activated by external stimuli to promote polymerization and rearrangement of the actin cytoskeleton, especially the actin c ...

, which appears to link

tyrosine kinase A tyrosine kinase is an enzyme that can transfer a phosphate group from ATP to the tyrosine residues of specific proteins inside a cell. It functions as an "on" or "off" switch in many cellular functions. Tyrosine kinases belong to a larger cla ...

signalling to cytoskeletal reorganization in the lamellipodium and its associated structures. Rac and Cdc42 are two

Rho Rho (uppercase Ρ, lowercase ρ or ; el, ρο or el, ρω, label=none) is the 17th letter of the Greek alphabet. In the system of Greek numerals it has a value of 100. It is derived from Phoenician letter res . Its uppercase form uses the sa ...

-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 pro ...

s which are normally

cytosol The cytosol, also known as cytoplasmic matrix or groundplasm, is one of the liquids found inside cells (intracellular fluid (ICF)). It is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondri ...

ic but can also be found in the cell membrane under certain conditions. When Cdc42 is activated, it can interact with

Wiskott–Aldrich syndrome protein The Wiskott–Aldrich Syndrome protein (WASp) is a 502-amino acid protein expressed in cells of the hematopoietic system that in humans is encoded by the ''WAS'' gene. In the inactive state, WASp exists in an autoinhibited conformation with sequ ...

(WASp) family receptors, in particular

N-WASp Neural Wiskott-Aldrich syndrome protein is a protein that in humans is encoded by the ''WASL'' gene. The Wiskott-Aldrich syndrome (WAS) family of proteins share similar domain structure, and are involved in transduction of signals from receptors ...

, which then activates Arp2/3. This stimulates actin branching and increases cell

motility Motility is the ability of an organism to move independently, using metabolic energy. Definitions Motility, the ability of an organism to move independently, using metabolic energy, can be contrasted with sessility, the state of organisms th ...

. 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