It is a cytoplasm-filled temporary projection of a eukaryotic cell membrane or a single-celled protist.
A pseudopod or pseudopodium (plural: pseudopods) (from the Greek word ψευδοποδός, ψευδός “false” + ποδός “foot”) can be used for motility or to ingest nutrients or other particles.
The cells that make pseudopods are generally called amoeboids .
An amoeba is a term generally used to describe a unicellular eukaryotic organism that does not have a defined shape and that moves through pseudopods.
Pseudopods are temporary projections of the cell, and the word literally means “false feet.”
The cell uses the pseudopods as a means of locomotion. The plural of amoeba is “amoeba”, not “amoebas.”
Although the word “amoeba” or “amoeboid” is often used to refer to all protozoa that move using pseudopods, the word Amoeba refers to a specific genus of protozoa.
The cytoplasm of an amoeba contains organelles and is enclosed by a cell membrane . An amoeba uses a process called phagoctiosis to obtain food.
This is a process in which the projections of the amoeba’s cell membrane extend and surround the food particle, completely enveloping it.
The food particle is internalized in a kind of “bubble” called a vacuole. The food particle can then be digested in the vacuole. The amoeba reproduces through mitosis.
Like many other protists, the structure of Amoeba proteus is relatively simple.
It is a single-celled organism that appears transparent and similar to gelatin, with a shape possibly “changing” forever, with a nucleus and organelles attached to the membrane (such as food vacuoles, contractile vacuoles, Golgi apparatus, mitochondria, etc.) .
Amoeba proteus does not have a fixed shape: it is constantly changing as it extends its pseudopods for motility and engulfs its prey. The average size of A. proteus is 500-1000 μm, visible with the naked eye!
Despite its small size compared to humans, A. proteus is considered a very large single-celled organism; in fact, they are closely related to the ‘giant amoebas’, which typically vary between 1000-3000 μm in size.
Many other single-celled protists and eukaryotes are microscopic and pale compared to the size of A. proteus.
Pseudopodia expand and contract through the reversible assembly of actin subunits in many microfilaments.
The filaments near the end of the cell interact with the myosin causing the contraction. The pseudopod extends until actin reassembles into a network.
Actin polymerization is assumed to be the source of the force that propels the cell forward.
Generally, several pseudopods arise from the surface of the body (polyporial, for example, Amoeba proteus), or a single pseudopod may form on the surface of the body (monopodial, for example, Entamoeba histolytica).
The cell surface projects a membrane process called lamellipodium, which is held inside by filaments that form at the leading edge, becoming webs as they fuse.
The cytoplasm flows into the lamellipodium, forming the pseudopods.
Pseudopods are critical for detecting prey that can then be eaten; enveloping pseudopods are called phagocytosis pseudopods. A common example of this type of amoeboid cell is the human white blood cell.
Human mesenchymal stem cells are a good example of a cell type that uses pseudopods for locomotor reasons: these migratory cells are responsible for in-utero remodeling, for example, in the formation of the trilaminar germ disc during gastrulation.
Lobopodia is bulbous and amoebic. Filopodia are slender, thread-like, and are largely supported by microfilaments.
Reticulopodia are very complex and bear individual pseudopodia that form irregular networks.
Axopodia are of the phagocytosis type with long, thin pseudopods supported by arrays of complex microtubules enveloped with cytoplasm and respond rapidly to physical contact.
Types of pseudopods
The pseudopod comes in different shapes and types.
In fact, the organisms that are capable of producing these projections vary according to the classification to which they belong, so the structure and shape of the projections define their taxonomic characteristics, making the shapes of the projections unique.
For a better understanding, the different types of pseudopod forms are shown below.
Pseudopodia can be classified into several varieties, according to the number of projections (monopodia and polypodia), and according to their appearance:
Lobopodia (the lobose seudopods)
They are bulbous, short, and blunt in shape (finger-shaped pseudopod). These finger-shaped tubular pseudopods contain ectoplasm and endoplasm. They occur in Lobosa and other Amoebozoa, and in some Heterolobosea (Excavata).
Lobopodia refers to the most common type observed in nature. They are characterized by short, blunt, bulbous projections that contain both the endoplasm and ectoplasm of the body.
An example is the wolfish amoeba, which is considered the largest of all pseudopods.
The filopodia (or pseudopodia filose)
Characterized by having a thin, threadlike pseudopod, filopodia have the ability to branch or anastomose, with pointed ends, consisting mainly of ectoplasm.
These formations are compatible with microfilaments. This is observed in some animal cells, in part of Filosa (Rhizaria), in “Testaceafilosia” (an artificial group that includes Euglifida), in Vampyrellidae and Pseudosporida (Rhizaria) and in Nuclearida (Opisthokonta).
Interestingly, this type of filipodia has different variations.
For example, granulopodia is similar to filopodia, but features a granular structure called extrusomes that specializes in capturing prey rather than providing mobility.
Another variation of the granulopodia is the granuloreticulopodia which is a cross between filipodia and reticulopodia. It is common among Allogromia species, which is a type of single-celled eukaryote.
The reticulopodia (or reticular pseudopodia)
Imagine that pseudopods are used as a means of communicating with other cells. They are complex formations in which individual pseudopods mix and form irregular networks.
The main function of reticulopodia, also known as myxopodia, is the ingestion of food, with locomotion as a secondary function. Reticulopods are typical of Foraminifera, Chlorarachnea, Gromia, and Filoreta (Rhizaria).
Reticulopodia is a specialized pseudopod that communicates with another pseudopodium, thus creating a network called the reticulum.
Also called reticular pseudopodia, they create complex projections in which individual pseudopodia fuse together to create what appear to be irregular networks.
This type of pseudopodium also works in both food ingestion and locomotion. Examples of reticular pseudopodia are the Foramineferans.
Axopodia are pseudopods supported by microtubule arrays. The pseudopods are enveloped by cytoplasm; therefore it is used more for phagocytosis or ingestion of food particles.
The axopodia are primarily responsible for phagocytosis by rapidly retracting in response to physical contact.
This supposedly puts pressure on the propeller, as after sensory action has occurred, it then dies.
Mainly, these pseudopods are food gathering structures. They are seen in “Radiolaria” (an artificial group within Rhizaria) and “Heliozoa” (artificial as well).
Pseudopods have two main functions: (1) locomotion and (2) prey capture or food wrapping.
For example, the amoeba can crawl by expanding the cytoplasm and contracting the filaments. The pseudopodbulge outward from the edge of the cell to attract the entire organism as it proceeds.
On the other hand, it is also used in the capture and ingestion of prey. They are also used to ingest particles while providing mobility during foraging.
In this process, the needle projections surround the food particle to create a membrane-wrapped sac that peels off to create a food vacuole before the food is fully digested.
Details of this process
Pseudopods are the most defined structures of A. proteus and part of what makes the organism so fascinating. These “false feet” are used for movement and to engulf their prey, making it an essential part of their structure.
Needless to say, without these structures, A. proteus would have had to adapt using other means to move and obtain nutrients, or be wiped off the planet’s surface.
Amoeba proteus moves by extending its cytoplasm and appears to do so in a slow and gliding manner. These extensions of its cytoplasm are called pseudopods.
This form of movement by extension of the cytoplasm is called “amoeboid movement” and is a common method of movement in other cells.
As the amoeba moves toward its prey, its pseudopods spread out and engulf the prey. The formation of pseudopodia can be explained by the change in viscosity (sol-gel theory).
The food vacuoles within A. proteus are not organelles that are “concrete,” meaning they come and go. They are the result of phagocytosis, the process by which A. proteus engulfs its prey.
A food vacuole is basically a food storage unit for the amoeba and is formed only when the amoeba has engulfed its prey completely, then the digestive enzymes are released into the vacuole.
The contractile vacuole is basically a water bubble within the endoplasm of A. proteus. Its function is to regulate the water content of the cell.
It is also a means of excreting its waste from the cell (through the cell membrane) through diffusion. A. proteus regularly moves to the surface of the water, when it needs to discharge waste and excess water.
This is done through osmosis, where there is a semi-permeable membrane that allows the flow of materials through the cell. Without a doubt, it is a very important organelle with an essential function for the amoeba.
The nucleus of A. proteus is a membrane-bound organelle that houses most of the cell’s genetic information and controls the actions of the amoeba.
If the nucleus is somehow removed from the cell (ie by splicing the cell into 2 parts), the cell quickly dies. It is an essential part in the reproduction of the cell.
Cytoplasm and sensitivity
The cytoplasm (endoplasm is the cytoplasm within the inner part of the cell) is the gelatinous substance within A. proteus in which the organelles are suspended.
It is also the part of the cell that allows A. proteus to form its pseudopods and preforms their respective functions. The appearance of the cytoplasm in A. proteus is slightly granular, caused by small crystals in the cytoplasm.
A. proteus (and amoebae in general) does not have a nervous system or sensory organelles. Therefore, they depend exclusively on their cytoplasm for sensitivity. They respond to various stimuli, such as: thigmotaxis and phototaxis.
A. proteus is an organism that requires oxygen, like other aerobic eukaryotes. You get oxygen through cellular respiration, basically the intake of oxygen and the production of carbon dioxide.
These gases simply enter and leave A. proteus through diffusion, through its semipermeable membrane.
This is a very thin membrane, with good regenerative capacities and elasticity. It contains the inner part of the cell (organelles, cytoplasm, etc.) and is semipermeable.
It allows the movement of materials in and out of the cell (i.e., water, oxygen, debris, etc.), making it an important component of the cell.
One characteristic that the plasma membrane of A. proteus has is the fact that it has many attached microvilli (can be seen under an electron microscope), which prevent the amoeba from sticking to the surface of the water.
When A. proteus “detects” unpleasant conditions, such as a nutrient-deficient environment, it removes its pseudopods and releases a protective covering over its plasma membrane (made of a chitin-like substance) called a cyst.
This process often leads to one of the ways that A. proteus reproduces: multi fission. This is one of the ways that A. proteus protects itself and ensures that it can reproduce under less than ideal conditions.
Examples of pseudopods
Several genera of the Kingdom Protista use pseudopods for mobility and ingestion. Protists are not classified into either plants or animals and have distinctive characteristics that make them worthy of having their own kingdom reserved for them.
Below are examples of pseudopods.
Pseudopods are a feature of a group of protozoan organisms called riopods under the Protista kingdom. They are characterized as eukaryotic cells that depend on the pseudopod for mobility.
They also use their pseudopod to wrap food particles inside a vacuole. Examples of rhizopods include Amoeba proteus, Entamoeba histolytica, Radiolarians, and Foramineferans.
These riopods are biologically significant. For example, Feramineferans skeletons comprise most of the chalk and limestone on the planet, while Entamoeba histolytica causes amoebic dysentery.
White blood cells
White blood cells are essential components of our immune system. Also called leukocytes, they help the body fight infection by attacking bacteria, viruses, and other pathogenic organisms that invade the body.
Phagocytic white blood cells, such as monocytes and neutrophils, form pseudopods to attack and absorb pathogens. The pseudopod also helps white blood cells to crawl within the body as excitable systems.
This means that the movement of the white blood cells is diverted to an area where more white blood cells are likely to attack a specific infection.