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A pseudopod or pseudopodium (plural: pseudopods or pseudopodia) is a temporary arm-like projection of a
eukaryotic Eukaryotes () are organisms whose Cell (biology), cells have a cell nucleus, nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the ...
cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment ( ...
that is emerged in the direction of movement. Filled with
cytoplasm In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. ...
, pseudopodia primarily consist of actin filaments and may also contain microtubules and intermediate filaments. Pseudopods are used for
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 ...
and
ingestion Ingestion is the consumption of a substance by an organism. In animals, it normally is accomplished by taking in a substance through the mouth into the gastrointestinal tract, such as through eating or drinking. In single-celled organisms in ...
. They are often found in
amoebas An amoeba (; less commonly spelled ameba or amœba; plural ''am(o)ebas'' or ''am(o)ebae'' ), often called an amoeboid, is a type of cell or unicellular organism with the ability to alter its shape, primarily by extending and retracting pseudopo ...
. Different types of pseudopodia can be classified by their distinct appearances.
Lamellipodia The lamellipodium (plural lamellipodia) (from Latin ''lamella'', related to ', "thin sheet", and the Greek radical ''pod-'', "foot") is a cytoskeletal protein actin projection on the leading edge of the cell. It contains a quasi-two-dimensiona ...
are broad and thin.
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 ...
are slender, thread-like, and are supported largely by microfilaments. Lobopodia are bulbous and amoebic. Reticulopodia are complex structures bearing individual pseudopodia which form irregular nets. Axopodia are the phagocytosis type with long, thin pseudopods supported by complex microtubule arrays enveloped with cytoplasm; they respond rapidly to physical contact. Some pseudopodial cells are able to use multiple types of pseudopodia depending on the situation: Most of them use a combination of lamellipodia and filopodia to migrate (such as metastatic cancer cells). The human foreskin fibroblasts can either use lamellipodia- or lobopodia-based migration in a 3D matrix depending on the matrix elasticity. Generally, several pseudopodia arise from the surface of the body, (''polypodial'', for example, ''
Amoeba proteus ''Amoeba proteus'' is a large species of amoeba closely related to another genus of giant amoebae, '' Chaos''. As such, the species is sometimes given the alternative scientific name ''Chaos diffluens''. This protozoan uses extensions called pseu ...
''), or a single pseudopod may form on the surface of the body (''monopodial'', such as ''
Entamoeba histolytica ''Entamoeba histolytica'' is an anaerobic parasitic amoebozoan, part of the genus ''Entamoeba''. Predominantly infecting humans and other primates causing amoebiasis, ''E. histolytica'' is estimated to infect about 35-50 million people worldwid ...
''). Cells which make pseudopods are generally referred to as
amoeboid An amoeba (; less commonly spelled ameba or amœba; plural ''am(o)ebas'' or ''am(o)ebae'' ), often called an amoeboid, is a type of cell or unicellular organism with the ability to alter its shape, primarily by extending and retracting pseudopo ...
s.


Formation


Via extracellular cue

To move towards a target, the cell uses chemotaxis. It senses extracellular signalling molecules, chemoattractants (e.g. cAMP for ''
Dictyostelium ''Dictyostelium'' is a genus of single- and multi-celled eukaryotic, phagotrophic bacterivores. Though they are Protista and in no way fungal, they traditionally are known as "slime molds". They are present in most terrestrial ecosystems ...
'' cells), to extend pseudopodia at the membrane area facing the source of these molecules. The chemoattractants bind to
G protein-coupled receptor G protein-coupled receptors (GPCRs), also known as seven-(pass)-transmembrane domain receptors, 7TM receptors, heptahelical receptors, serpentine receptors, and G protein-linked receptors (GPLR), form a large group of evolutionarily-related p ...
s, which activate GTPases of the Rho family (e.g. Cdc42, Rac) via
G-proteins G proteins, also known as guanine nucleotide-binding proteins, are a family of proteins that act as molecular switches inside cells, and are involved in transmitting signals from a variety of stimuli outside a cell to its interior. Their ac ...
. Rho GTPases are able to activate
WASp A wasp is any insect of the narrow-waisted suborder Apocrita of the order Hymenoptera which is neither a bee nor an ant; this excludes the broad-waisted sawflies (Symphyta), which look somewhat like wasps, but are in a separate suborder. ...
which in turn activate
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 ...
which serve as nucleation sites for actin polymerization. The actin polymers then push the membrane as they grow, forming the pseudopod. The pseudopodium can then adhere to a surface via its adhesion proteins (e.g.
integrin Integrins are transmembrane receptors that facilitate cell-cell and cell-extracellular matrix (ECM) adhesion. Upon ligand binding, integrins activate signal transduction pathways that mediate cellular signals such as regulation of the cell cycle ...
s), and then pull the cell's body forward via contraction of an actin-myosin complex in the pseudopod. This type of locomotion is called
Amoeboid movement Amoeboid movement is the most typical mode of locomotion in adherent eukaryotic cells. It is a crawling-like type of movement accomplished by protrusion of cytoplasm of the cell involving the formation of pseudopodia ("false-feet") and posterior ...
. Rho GTPases can also activate
phosphatidylinositol 3-kinase Phosphoinositide 3-kinases (PI3Ks), also called phosphatidylinositol 3-kinases, are a family of enzymes involved in cellular functions such as cell growth, proliferation, differentiation, motility, survival and intracellular trafficking, which i ...
(PI3K) which recruit PIP3 to the membrane at the leading edge and detach the PIP3-degrading enzyme PTEN from the same area of the membrane. PIP3 then activate GTPases back via
GEF Gef ( ), also referred to as the Talking Mongoose or the Dalby Spook, was the name given to an allegedly talking mongoose which was claimed to inhabit a farmhouse owned by the Irving family. The Irvings' farm was located at Cashen's Gap near ...
stimulation. This serves as a feedback loop to amplify and maintain the presence of local GTPase at the leading edge. Otherwise, pseudopodia can't grow on other sides of the membrane than the leading edge because myosin filaments prevent them to extend. These myosin filaments are induced by
cyclic GMP Cyclic guanosine monophosphate (cGMP) is a cyclic nucleotide derived from guanosine triphosphate (GTP). cGMP acts as a second messenger much like cyclic AMP. Its most likely mechanism of action is activation of intracellular protein kinases in r ...
in '' D. discoideum'' or
Rho kinase Rho-associated protein kinase (ROCK) is a kinase belonging to the AGC (PKA/ PKG/PKC) family of serine-threonine specific protein kinases. It is involved mainly in regulating the shape and movement of cells by acting on the cytoskeleton. ROCKs ( ...
in
neutrophil Neutrophils (also known as neutrocytes or heterophils) are the most abundant type of granulocytes and make up 40% to 70% of all white blood cells in humans. They form an essential part of the innate immune system, with their functions varying ...
s for example. Different physical parameters were shown to regulate the length and time-scale of pseudopodia formation. For example, an increase in membrane
tension Tension may refer to: Science * Psychological stress * Tension (physics), a force related to the stretching of an object (the opposite of compression) * Tension (geology), a stress which stretches rocks in two opposite directions * Voltage or el ...
inhibits actin assembly and protrusion formation. It was demonstrated that the lowered negative surface charge on the inner surface of the plasma membrane generates protrusions via activation of the Ras- PI3K/AKT/mTOR signalling pathway.


Without extracellular cue

In the case there is no extracellular cue, all moving cells navigate in random directions, but they can keep the same direction for some time before turning. This feature allows cells to explore large areas for colonization or searching for a new extracellular cue. In ''Dictyostelium'' cells, a pseudopodium can form either ''de novo'' as normal, or from an existing pseudopod, forming a Y-shaped pseudopodium. The Y-shaped pseudopods are used by ''Dictyostelium'' to advance relatively straight forward by alternating between retraction of the left or right branch of the pseudopod. The ''de novo'' pseudopodia form at different sides than pre-existing ones, they are used by the cells to turn. Y-shaped pseudopods are more frequent than ''de novo'' ones, which explain the preference of the cell to keep moving to the same direction. This persistence is modulated by
PLA2 The enzyme phospholipase A2 (EC 3.1.1.4, PLA2, systematic name phosphatidylcholine 2-acylhydrolase) catalyse the cleavage of fatty acids in position 2 of phospholipids, hydrolyzing the bond between the second fatty acid “tail” and the glycer ...
and cGMP signalling pathways.


Functions

The functions of pseudopodia include locomotion and ingestion: * Pseudopodia are critical in sensing targets which can then be engulfed; the engulfing pseudopodia are called
phagocytosis Phagocytosis () is the process by which a cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs phagocytosis is ...
pseudopodia. A common example of this type of amoeboid cell is the macrophage. * They are also essential to amoeboid-like locomotion. Human
mesenchymal stem cell Mesenchymal stem cells (MSCs) also known as mesenchymal stromal cells or medicinal signaling cells are multipotent stromal cells that can differentiate into a variety of cell types, including osteoblasts (bone cells), chondrocytes (cartilage cel ...
s are a good example of this function: these migratory cells are responsible for in-utero remodeling; for example, in the formation of the
trilaminar germ disc A trilaminar embryonic disc (or trilaminary blastoderm, or trilaminar germ disk) is an early stage in the development of triploblastic organisms, which include humans and many other animals. It is the next stage from the earlier bilaminar embryo ...
during gastrulation.


Morphology

Pseudopods can be classified into several varieties according to the number of projections (monopodia and polypodia), and according to their appearance:


Lamellipodia

Lamellipodia The lamellipodium (plural lamellipodia) (from Latin ''lamella'', related to ', "thin sheet", and the Greek radical ''pod-'', "foot") is a cytoskeletal protein actin projection on the leading edge of the cell. It contains a quasi-two-dimensiona ...
are broad and flat pseudopodia used in locomotion. They are supported by microfilaments which form at the leading edge, creating a mesh-like internal network.


Filopodia

Filopodia (or filose pseudopods) are slender and filiform with pointed ends, consisting mainly of ectoplasm. These formations are supported by
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 ...
s which, unlike the filaments of lamellipodia with their net-like actin, form loose bundles by
cross-link In chemistry and biology a cross-link is a bond or a short sequence of bonds that links one polymer chain to another. These links may take the form of covalent bonds or ionic bonds and the polymers can be either synthetic polymers or natural ...
ing. This formation is partly due to bundling proteins such as
fimbrin Fimbrin also known as is plastin 1 is a protein that in humans is encoded by the PLS1 gene. Fimbrin is an actin cross-linking protein important in the formation of filopodia. Structure Fimbrin belongs to the calponin homology (CH) domain supe ...
s and
fascin Fascin is an actin bundling protein. Species and tissue distribution It is a 54-58 kilodalton monomeric actin filament bundling protein originally isolated from sea urchin egg but also found in ''Drosophila'' and vertebrates, including ...
s. Filopodia are observed in some animal cells: in part of Filosa (
Rhizaria The Rhizaria are an ill-defined but species-rich supergroup of mostly unicellular eukaryotes. Except for the Chlorarachniophytes and three species in the genus Paulinella in the phylum Cercozoa, they are all non-photosynthethic, but many forami ...
), in "
Testaceafilosia Testate amoebae (formerly thecamoebians, Testacea or Thecamoeba) are a polyphyletic group of unicellular amoeboid protists, which differ from naked amoebae in the presence of a test (biology), test that partially encloses the cell, with an apertur ...
" , in
Vampyrellidae The family Vampyrellidae is a subgroup of the order Aconchulinida (formerly Vampyrellida) within the phylum Cercozoa. Based on molecular sequence data, the family currently comprises the genus ''Vampyrella'', and maybe several other vampyrellid ...
and Pseudosporida (
Rhizaria The Rhizaria are an ill-defined but species-rich supergroup of mostly unicellular eukaryotes. Except for the Chlorarachniophytes and three species in the genus Paulinella in the phylum Cercozoa, they are all non-photosynthethic, but many forami ...
) and in
Nucleariida Nucleariida is a group of amoebae with filose pseudopods, known mostly from soils and freshwater. They are distinguished from the superficially similar vampyrellids mainly by having mitochondria with discoid cristae, in the absence of superf ...
( Opisthokonta).


Lobopodia

Lobopodia (or lobose pseudopods) are bulbous, short, and blunt in form. These finger-like, tubular pseudopodia contain both ectoplasm and
endoplasm Endoplasm generally refers to the inner (often granulated), dense part of a cell's cytoplasm. This is opposed to the ectoplasm which is the outer (non-granulated) layer of the cytoplasm, which is typically watery and immediately adjacent to the ...
. They can be found in different kind of cells, notably in
Lobosa Lobosa is a taxonomic group of amoebae in the phylum Amoebozoa. Most lobosans possess broad, bluntly rounded pseudopods, although one genus in the group, the recently discovered ''Sapocribrum,'' has slender and threadlike (filose) pseudopodia. ...
and other Amoebozoa and in some
Heterolobosea The Percolozoa are a group of colourless, non-photosynthetic Excavata, including many that can transform between amoeboid, flagellate, and cyst stages. Characteristics Most Percolozoa are found as bacterivores in soil, fresh water and occasionall ...
(
Excavata Excavata is a major supergroup of unicellular organisms belonging to the domain Eukaryota. It was first suggested by Simpson and Patterson in 1999 and introduced by Thomas Cavalier-Smith in 2002 as a formal taxon. It contains a variety of free- ...
). High-pressure lobopodia can also be found in human fibroblasts travelling through a complex network of 3D
matrix Matrix most commonly refers to: * ''The Matrix'' (franchise), an American media franchise ** ''The Matrix'', a 1999 science-fiction action film ** "The Matrix", a fictional setting, a virtual reality environment, within ''The Matrix'' (franchis ...
(e.g. mammalian dermis, cell-derived matrix). Contrarily to other pseudopodia using the pressure exerted by actin polymerization on the membrane to extend, fibroblast lobopods use the nuclear piston mechanism consisting in pulling the nucleus via actomyosin contractility to push the cytoplasm that in turn push the membrane, leading to pseudopod formation. To occur, this lobopodia-based fibroblast migration needs nesprin 3,
integrin Integrins are transmembrane receptors that facilitate cell-cell and cell-extracellular matrix (ECM) adhesion. Upon ligand binding, integrins activate signal transduction pathways that mediate cellular signals such as regulation of the cell cycle ...
s,
RhoA Transforming protein RhoA, also known as Ras homolog family member A (RhoA), is a small GTPase protein in the Rho family of GTPases that in humans is encoded by the ''RHOA'' gene. While the effects of RhoA activity are not all well known, it is ...
,
ROCK Rock most often refers to: * Rock (geology), a naturally occurring solid aggregate of minerals or mineraloids * Rock music, a genre of popular music Rock or Rocks may also refer to: Places United Kingdom * Rock, Caerphilly, a location in Wales ...
and
myosin II Myosins () are a superfamily of motor proteins best known for their roles in muscle contraction and in a wide range of other motility processes in eukaryotes. They are ATP-dependent and responsible for actin-based motility. The first myosin (M2 ...
. Otherwise, lobopods are often accompanied with small lateral blebs forming along the side of the cell, probably due to the high intracellular pressure during lobopodia formation increasing the frequency of plasma membrane-cortex rupture.


Reticulopodia

Reticulopodia (or reticulose pseudopods), are complex formations in which individual pseudopods are merged and form irregular nets. The primary function of reticulopodia, also known as myxopodia, is food ingestion, with locomotion a secondary function. Reticulopods are typical of
Foraminifera Foraminifera (; Latin for "hole bearers"; informally called "forams") are single-celled organisms, members of a phylum or class of amoeboid protists characterized by streaming granular ectoplasm for catching food and other uses; and commonly ...
, Chlorarachnea, ''
Gromia ''Gromia'' is a genus of protists, closely related to foraminifera, which inhabit marine and freshwater environments. It is the only genus of the family Gromiidae. ''Gromia'' are ameboid, producing filose pseudopodia that extend out from the ce ...
'' and '' Filoreta'' (Rhizaria).


Axopodia

Axopodia (also known as actinopodia) are narrow pseudopodia containing complex arrays of microtubules enveloped by cytoplasm. Axopodia are mostly responsible for phagocytosis by rapidly retracting in response to physical contact. Principally, these pseudopodia are food collecting structures. Also used in passive transportation by expanding their surface area , and being carried with the water stream. They are observed in "
Radiolaria The Radiolaria, also called Radiozoa, are protozoa of diameter 0.1–0.2 mm that produce intricate mineral skeletons, typically with a central capsule dividing the cell into the inner and outer portions of endoplasm and ectoplasm. The el ...
" and "
Heliozoa Heliozoa, commonly known as sun-animalcules, are microbial eukaryotes ( protists) with stiff arms ( axopodia) radiating from their spherical bodies, which are responsible for their common name. The axopodia are microtubule-supported projections f ...
".


References

{{Authority control Actin-based structures Cell anatomy Cell movement