Amoeboid: Definition, Characteristics, Examples, Form and Modalities of Amoeboid Movement

It is a type of cell or organism that has the ability to alter its shape, mainly by spreading and retracting pseudopods.

An amoeba is often called an amoeboid.

Amoebas do not form a single taxonomic group; instead, they are found in all major lineages of eukaryotic organisms . Amoeboid cells occur not only among protozoa, but also in fungi, algae, and animals.

Microbiologists often use the terms “amoeboid” and “amoeba” interchangeably for any organism that exhibits amoeboid movement.

In the oldest classification systems, most amoebae were placed in the class or subphylum Sarcodina, a grouping of single-celled organisms that possess pseudopods or move by protoplasmic flow.

However, molecular phylogenetic studies have shown that Sarcodine is not a monophyletic group whose members share common descent. Consequently, amoeboid organisms are no longer classified together in a group.

The best known amoeboid protists are the “giant amoebas” Chaos carolinense and Amoeba proteus, which have been widely cultivated and studied in classrooms and laboratories.

Other well-known species include the so-called “brain-eating amoeba” Naegleria fowleri, the intestinal parasite Entamoeba histolytica, which causes amoebic dysentery, and the multicellular “social amoeba” or slime mold Dictyostelium discoideum.

Shape and nutrition

The appearance and internal structure of pseudopods are used to distinguish groups of amoebae from each other.

Amebozoan species, such as those of the genus Amoeba, typically have bulbous (lobular) pseudopods, rounded at the ends and roughly tubular in cross section.

Cercozoan amoeboids, such as Euglypha and Gromia, have slender, pinkish pseudopods (filose). Foraminifera emit fine, branched pseudopods that fuse together to form lattice-like (reticulous) structures.

Some groups, such as Radiolaria and Heliozoa, have rigid, needle-like, radiating axopodia (actinopoda) supported from within by bundles of microtubules.

Free-living amoebae can be “tested” (enclosed within a hard shell), or “naked” (eg, gymnamoebae, without a hard shell).

The shells of the tested amoebae can be composed of various substances, including calcium, silica, chitin or clumps of found materials, such as small grains of sand and diatom frustics.

To regulate osmotic pressure, most freshwater amoebas have a contractile vacuole that expels excess water from the cell.

This organelle is necessary because fresh water has a lower concentration of solutes (such as salt) than the amoeba’s own internal fluids (cytosol).

Because the surrounding water is hypotonic with respect to the content of the cell, the water is transferred through the cell membrane of the amoeba by osmosis.

Without a contractile vacuole, the cell would overfill with water and eventually burst.

Marine amoebas generally do not possess a contractile vacuole because the concentration of solutes within the cell is in equilibrium with the tonicity of the surrounding water.

The food sources of amoebae vary. Some amoebas are predatory and live by consuming bacteria and other protists. Some are detritivores and eat dead organic material.

Amoebas typically ingest their food by phagocytosis, spreading pseudopodia to surround and engulf live prey or particles of sequestered material.

Amoeboid cells do not have a mouth or cytostome, and there is no fixed location in the cell where phagocytosis normally occurs.

Some amoebas also feed on pinocytosis, soaking up dissolved nutrients through vesicles formed within the cell membrane.

Amoeboid movement

Amoeboid movement is the most common mode of locomotion in eukaryotic cells. It is a type of movement that creeps through the protrusion of the cell’s cytoplasm, which involves the formation of pseudopods (“false feet”) and posterior uropods.

One or more pseudopods can be produced at the same time depending on the organism, but all the movements of amoeboids are characterized by the movement of organisms with an amorphous shape that do not have established mobility structures.

The movement occurs when the cytoplasm slides and forms a pseudopodium in front to propel the cell forward.

This type of movement has been linked to changes in action potential, although the exact mechanism is still unknown.

Some examples of organisms that exhibit this type of locomotion are amoeboids, slime molds, and some protozoa such as Naegleria gruberi, as well as some cells in humans such as leukocytes.

Sarcomas, or cancers that arise from connective tissue cells, are particularly adept at amoeboid movement, leading to their high rate of metastasis.

While several hypotheses have been proposed to explain the mechanism of amoeboid movement, the exact mechanism is still unknown.

Size range

The size of the cells and amoeboid species is extremely variable.

The marine amoeboid Massisteria voersi is only 2.3 to 3 microns in diameter, within the size range of many bacteria.

At the other extreme, the shells of deep-sea xenophyophores can reach 20 cm in diameter.

Most of the free-living freshwater amoebas commonly found in pond, ditch and lake water are microscopic, but some species, such as the so-called “giant amoebas” Pelomyxa palustris and Chaos carolinense, can be large enough to be seen. naked eye.

Amoebas as specialized cells and life cycle stages

Some multicellular organisms have amoeboid cells only in certain phases of life, or use amoeboid movements for specialized functions.

In the immune system of humans and other animals, amoeboid white blood cells pursue invading organisms, such as bacteria and pathogenic protists, and engulf them by phagocytosis.

Amoeboid stages also occur in fungal-like multicellular protists, so-called slime molds.

Both the plasmodial slime molds, currently classified in the Myxogastria class, and the cellular slime molds of the Acrasida and Dictyosteliida groups, live as amoebae during their feeding stage.

The amoeboid cells of the former combine to form a giant multinucleated organism, while the cells of the latter live separately until food runs out, at which point the amoebae aggregate to form a multicellular migratory ‘slug’ that functions as a single organism.

Other organisms can also have amoeboid cells during certain stages of the life cycle, for example:

The gametes of some green algae (Zygnematophyceae) and diatoms of Pennate, the spores (or dispersal phases) of some Mesomycetozoea, and the sporoplasm stage of Myxozoa and Ascetosporea.

Early history and origins of Sarcodina

The oldest record of an amoeboid organism was produced in 1755 by August Johann Rösel von Rosenhof, who called his discovery “Der Kleine Proteus” (“the little Proteus”).

Rösel’s illustrations show an unidentifiable freshwater amoeba, similar in appearance to the common species now known as Amoeba proteus.

The term “Proteus animalcule” remained in use throughout the 18th and 19th centuries, as an informal name for any large, free-living amoeboid.

In 1822, the genus Amoeba (from the Greek ἀμοιβή amoibe, meaning “change”) was erected by the French naturalist Bory de Saint-Vincent. Bory’s contemporary, CG Ehrenberg, adopted the genus in his own classification of microscopic creatures, but changed the spelling to Amoeba.

In 1841, Felix Dujardin coined the term “sarcode” (from the Greek σάρξ sarx, “meat” and εἶδος eidos, “form”) for the “thick, glutinous and homogeneous substance” that fills the bodies of protozoal cells.

Although the term originally referred to the protoplasm of any protozoan, it soon came to be used in a restricted sense to designate the gelatinous contents of amoeboid cells.

Thirty years later, Austrian zoologist Ludwig Karl Schmarda used ‘sarcode’ as the conceptual basis for his division Sarcodea, a phyl-level group made up of ‘unstable, changeable’ organisms with bodies largely composed of ‘sarcode’.

Later workers, including the influential taxonomist Otto Bütschli, amended this group to create the class Sarcodina, a taxon that remained in wide use for most of the 20th century.

Within the traditional Sarcodina, amoebas were generally divided into morphological categories, based on the shape and structure of their pseudopods.

Amoebas with pseudopods supported by regular microtubule arrangements (such as freshwater Heliozoa and Radiolaria marina) were classified as Actinopods; while those with unsupported pseudopods were classified as Rhizopods.

Rhizopods were subdivided into lobose, phylum, and reticulose amoebas, according to the morphology of their pseudopods.

Sarcodina dismantling

In the last decade of the 20th century, a series of molecular phylogenetic analyzes confirmed that Sarcodine was not a monophyletic group.

In view of these findings, the old scheme was abandoned and the Sarcodina amoebas dispersed among many other high-level taxonomic groups.

Today, most traditional sarcodines are placed in two supergroups of eukaryotes: Amoebozoa and Rhizaria.

The rest has been distributed among the excavations, opisthokonts and stramenopiles. Some, like the Centrohelida, have yet to be placed in any supergroups.

Pathogenic interactions with other organisms

Some amoebae can infect other organisms in a pathogenic way, causing diseases:

Entamoeba histolytica is the cause of amoebiasis or amoebic dysentery. Naegleria fowleri (the “brain-eating amoeba”) is a native freshwater species that can be fatal to humans if introduced through the nose.

Acanthamoeba can cause amoebic keratitis and encephalitis in humans. Balamuthia mandrillaris is the cause of granulomatous amoebic meningoencephalitis (often fatal)

The amoeba has been found to harvest and grow the bacteria involved in plague.

Mitosis

Recent evidence indicates that several Amoebozoa lineages suffer from meiosis.

The orthologs of genes used in sexual eukaryotic meiosis have recently been identified in the Acanthamoeba genome. These genes include: Spo11, Mre11, Rad50, Rad51, Rad52, Mnd1, Dmc1, Msh, and Mlh.

This finding suggests that “Acanthamoeba” is capable of some form of meiosis and may be capable of undergoing sexual reproduction.

The meiosis-specific recombinase, Dmc1, is required for efficient meiotic homologous recombination, and Dmc1 is expressed in Entamoeba histolytica.

Purified Dmc1 from E. histolytica forms presynaptic strands and catalyzes homologous adenosine triphosphate DNA pairing and DNA strand swapping over at least several thousand base pairs.

DNA pairing and chain exchange reactions are enhanced by the eukaryotic meiosis-specific (heterodimer) accessory recombination factor Hop2-Mnd1.

These processes are central to meiotic recombination, suggesting that E. histolytica undergoes meiosis.

Studies of Entamoeba invadens found that, during the conversion of the tetraploid uninucleated trophozoite to the tetranucleated cyst, homologous recombination is enhanced.

The expression of genes with functions related to the main steps of meiotic recombination also increases during encystations. These findings in E. invadens, combined with evidence from E. histolytica studies indicate the presence of meiosis in Entamoeba.

Dictyostelium discoideum in the supergroup Amoebozoa can undergo sexual reproduction and reproduction, including meiosis when food is scarce.

Since Amoebozoa diverged early from the eukaryotic family tree, these results suggest that meiosis was present early in eukaryotic evolution.

Furthermore, these findings are consistent with the proposal by Lahr et al. that most amoeboid lineages are formerly sexual.

In popular culture

The crew of the Enterprise faced a giant amoeba in the 1968 episode of Star Trek, “The Immunity Syndrome.”

“A Very Cellular Song”, a song by the 1968 British psychedelic folk band Incredible String Band, The Hangman’s Beautiful Daughter, is partially told from the point of view of an amoeba.

“Amoeba”, a song from the 1981 debut album The Adolescents, by the American punk band The Adolescents.

Molecular mechanism of cell movement

Sol-gel theory

The protoplasm of an amoeba is made up of an outer layer called ectoplasm that surrounds an inner portion called endoplasm.

Ectoplasm consists of a semi-solid gelatinous called plasma gel while endoplasm is composed of a less viscous fluid called plasma sol.

Ectoplasm owes its highly viscous state, in part, to the crosslinking actomyosin complex it contains.

The locomotion of an amoeba is believed to occur due to the sol-gel conversion of the protoplasm within its cell. Sol-gel conversion describes the contraction and relaxation events that are imposed by osmotic pressure and other ionic charges.

For example, when an amoeba moves, it spreads a gelatinous, cytosolic pseudopodium, which then results in the more fluid cytosol (plasma sol) flowing past the gelatinous portion (plasma gel) where it freezes at the end of the pseudopodium.

This results in the extension of this appendix. At the opposite (back) end of the cell, the plasma gel is then converted to plasma sol and transmitted by airflow to the advancing pseudopodium.

As long as the cell has a way to deal with the substrate, repeating this process guides the cell forward. Inside the amoeba, there are proteins that can be activated to turn the gel into a more liquid, sunny state.

The cytoplasm consists primarily of actin and actin is regulated by actin-binding proteins.

Actin-binding proteins are in turn regulated by calcium ions; therefore, calcium ions are very important in the sol-gel conversion process.

Modalities of amoeboid movement

Actin-driven motility

Based on some mathematical models, recent studies hypothesize a new biological model for the collective biomechanical and molecular mechanisms of cell movement.

It is proposed that microdomains weave the texture of the cytoskeleton and their interactions mark the location for the formation of new adhesion sites.

According to this model, microdomain signaling dynamics organizes the cytoskeleton and its interaction with the substrate.

As the microdomains activate and maintain the active polymerization of actin filaments, their propagation and zigzag movement in the membrane generate a highly interconnected network of linear or curved filaments oriented at a wide spectrum of angles with respect to the cell boundary. .

It has also been proposed that the interaction of microdomains marks the formation of new focal adhesion sites in the cell periphery.

The interaction of myosin with the actin network generates membrane shrinkage / undulation, retrograde flow, and contractile forces for forward movement.

Finally, the continuous application of stress to the old focal adhesion sites could result in calcium-induced activation of calpain and, consequently, the loosening of focal adhesions that completes the cycle.

In addition to actin polymerization, microtubules can also play an important role in cell migration where lamellipodia formation is involved.

An experiment showed that although microtubules are not required for actin polymerization to create lamellipodial extensions, they are needed to allow cell movement.

Blister-driven motility

Another proposed mechanism of this type, the blister-driven mechanism of amoeboid locomotion, suggests that the actomyosin cortex contracts to increase hydrostatic pressure within the cell.

Blistering occurs in amoeboid cells when there is an approximately spherical bulge on the cell membrane characterized by separation of the actomyosin cortex.

This amoeboid mode of movement requires myosin II to play a role in generating the hydrostatic pressure that causes the bleb to expand.

This is different from actin-driven locomotion, where the protrusion created is by polymerization of actin while remaining attached to the actomyosin cortex and physically pushing against the cell’s barrier.

During blister-driven movement of amoeboids, the cytoplasmic sol-gel state is regulated.

Blebbing can also be a sign of when a cell is undergoing apoptosis.

Mobile cell blisters have also been observed to undergo an approximately uniform life cycle lasting approximately one minute.

This includes a phase that involves the initial outward expansion where the membrane separates from the membranous cytoskeleton.

This is followed by a brief static phase where the hydrostatic pressure that has built up is sufficient to maintain the size of the blister.

This is followed by the last phase characterized by slow retraction of the bleb and reintroduction of the membrane into the cytoskeletal infrastructure.

Cells can undergo rapid transitions between blistering and lamellipodium-based motility as a means of migration.

However, the speed at which these transitions are made is still unknown. Tumor cells can also exhibit rapid transitions between amoeboid motility and mesenchymal motility, another form of cell movement.

Related movement mechanisms

Dictyostelium cells and neutrophils can also swim, using a mechanism similar to crawling.

Another form of unicellular movement that is displayed in Euglena is known as metabolism.

Characteristics of amoeboid cells

The reason amoebae and amoeboid cells are able to change shape has to do with the cytoplasm contained in the cell and the parts of the cell skeleton known as the cytoskeleton.

The cytoplasm contains a substance known as plasmagel that can change consistency and move more than the rest of the cytoplasm. The cytoskeleton contains parts that can contract and expand.

Contraction and expansion, together with the changing consistency of plasmagel, change the shape of cells.

The ability to change shape is just one characteristic exhibited by amoeboid cells. They have a modified movement method based on the ability to shapeshift.

Amoeboid cells use what is called a pseudopod to advance. Now whenever you see that pseudo- prefix, you know it means false, similar or similar.

In other words, it is almost, but not exactly, what its name says. In the case of pseudopods, this means ‘false foot’ or ‘like a foot’.

Amoeboid cells change shape to protrude from part of the cell, and it almost functions like a foot. The rest of the cell will go in the direction of the protrusion. This type of movement is known as amoeboid movement.

Amoeboid cells can also carry out a process known as phagocytosis, which literally means eating or ingesting other cells.

Amoeboid cells achieve phagocytosis by using pseudopods to surround and engulf other cells. Once the cells are engulfed, they are digested by the amoeboid cells.

The constant change in shape makes amoeboid cells asymmetric. This means that they do not show any symmetry at all.

Examples of amoeboid cells

So now you know what amoeboid cells are, but you may be trying to figure out which cells are classified as amoeboid cells. Well, there are some who are working to defend your body right now.

We all have white blood cells in our bodies that are amoeboid cells. Macrophages, the white blood cells that use phagocytosis to rid the body of foreign cells, are amoeboid cells.

They change their shape to engulf bacteria and other invaders in the body in order to defend the body against disease and infection.