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A pseudopod or pseudopodium (plural: pseudopods or pseudopodia) (from the Greek word ψευδοποδός, ψευδός "false" + ποδός "foot") is a temporary cytoplasm-filled projection of an eukaryotic cell membrane. They primarily consist of actin filaments and may also contain microtubules and intermediate filaments. Pseudopods are used for motility and ingestion.

Different types of pseudopodia can be classified by their distinct appearances. Lobopodia are bulbous and amoebic. Filopodia are slender, thread-like, and are supported largely by microfilaments. 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. Lamellipodia are broad and thin.

Generally, several pseudopodia arise from the surface of the body, (polypodial, e.g. Amoeba proteus), or, a single pseudopod may form on the surface of the body (monopodial, e.g. Entamoeba histolytica).

Cells which make pseudopods are generally referred to as amoeboids.

Via extracellular cue
To move towards a target, the cell uses chemotaxis. It senses extracellular signalling molecules, chemoattractants (e.g. cAMP for Dictyostelium cells ), to extend pseudopodia at the membrane area facing the source of these molecules.

The chemoattractants bind to G protein-coupled receptors, which activate GTPases of the Rho family (e.g. Cdc42, Rac) via G-proteins.

Rho GTPases are able to activate WASp which in turn activate Arp2/3 complex which serve as nucleation sites for actin polymerization. The actin polymers then push the membrane as they grow through actin polymerization. The pseudopodia can then adhere to a surface via its adhesion proteins (e.g. integrins), and then pull the body of the cell forward via contraction of an actin-myosin complex in the pseudopod. This type of locomotion is called Amoeboid movement.

Rho GTPases can also activate phosphatidylinositol 3-kinase (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 stimulation. This serves as a feedback loop to amplify and maintain the local GTPase at the leading edge.

Otherwise, pseudopodia can't grow at other places on the membrane than the leading edge because myosin filaments prevent them to extend. These myosin filaments are induced by cyclic GMP in D. discoideum and Rho kinase in neutrophils for example.

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 either form 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 are used by those cells to turn.

Y-shaped pseudopods are more frequent than de novo ones, thus the preference of the cell to keep moving to the same direction. This is modulated by PLA2 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 pseudopodia. A common example of this type of amoeboid cell is the macrophage.
 * They are also essential to amoeboid-like locomotion. Human mesenchymal stem cells 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 during gastrulation.

Morphology


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

Lobopodia
Lobopodia (or lobose pseudopods) are bulbous, short, and blunt in form. These finger-like, tubular pseudopodia contain both ectoplasm and endoplasm. They can be found in different kind of cells, notably in Lobosa and other Amoebozoa and in some Heterolobosea (Excavata).

High-pressure lobopodia can also be found in human fibroblasts travelling through a complex network of 3D matrix (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, thus forming the pseudopod. To occur, this lobopodia-based fibroblast migration needs nesprin 3, integrins, RhoA, ROCK and myosin II. 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.

Filopodia
Filopodia (or filose pseudopods) are slender and filiform with pointed ends, consisting mainly of ectoplasm. These formations are supported by microfilaments which, unlike the filaments of lamellipodia with their net-like actin, form loose bundles by cross-linking. This formation is partly due to bundling proteins such as fimbrins and fascins. Filopodia are observed in some animal cells: in part of Filosa (Rhizaria), in "Testaceafilosia", in Vampyrellidae and Pseudosporida (Rhizaria) and in Nucleariida (Opisthokonta).

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, Chlorarachnea, Gromia 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. They are observed in "Radiolaria" and "Heliozoa".

Lamellipodia
Lamellipodia 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.