Synonyms: pseudopods. The cell that forms pseudopodia is referred to as amoeba or amoeboid. The term amoeboid is used to indicate an amoeba-like cell, and thus, sets the latter apart from the true amoeba of the genus Amoeba. Looking at the structure of an amoeboid cell, one would find two major regions: the endoplasm and the ectoplasm. The endoplasm is the inner region that is granular and metabolically active whereas the ectoplasm is the outer region that is clear and contains large numbers of actin filaments.
The actin filaments in the ectoplasm are responsible for making the latter contractile and somewhat flexible. The actin filaments are a type of cytoskeleton that can be identified from the other types by being relatively thin with a diameter of about 7 nm and comprised of actin subunits especially F-actin proteins.
The filaments form from actin polymerization through the aid of assembly proteins, such as motor proteins, capping proteins, and branching proteins. Other cytoskeleton types found in the cytoplasmic projections are microtubules and intermediate filaments. Intermediate filaments are a type of cytoskeleton with diameters ranging from 8 to 12 nm. The actin filaments are the thinnest cytoskeleton among the three. In the cell body, pseudopodia may be formed when the actin proteins polymerize and form chains.
Cell protrusion is driven by a protrusive force by actin polymerization. Actins forming chains apparently provide the force that pushes the cell membrane to the direction of the movement.
When a projection is formed, the rest of the cytoplasm slides forward, thus, moving the cell forward. This form of locomotion is referred to as an amoeboid movement.
The direction may be determined by chemotaxis and formation may be impelled by the presence of chemical attractants. For instance, chemical attractants bind to G protein-coupled receptors of the cell membrane resulting in the activation of internal signal transduction pathways that ultimately lead to activating actin polymerization.
The formation of actin results in the cell forming pseudopodium toward the direction of the source. Pseudopodia may also form without an external cue. Amoeboid cells may also form several pseudopodia all at once. Furthermore, a pseudopod may form from another pseudopod, and thus resemble the letter Y. Apart from the actin filaments, there is growing evidence indicating that microtubules as well seem to play a role in pseudopod formation, e.
According to appearance, the types of pseudopodia are lobopodia bulbous , filopodia slender, thread-like , reticulopodia a network of pseudopods , axopodia thin pseudopods containing complex arrays of microtubules , and lamellipodia broad and flat pseudopodia.
Lobopodia are a type of pseudopodia characterized by the fingerlike, bulbous, bluntly rounded, tubular cytoplasmic projections. The pseudopod contains both ectoplasm and endoplasm. This type of pseudopodia is one of the distinctive features of the taxonomic group, Lobosa. They are also seen in certain Amoebozoa and Excavata.
In humans, fibroblasts are amoeboid cells that form lobopodia as they travel through the extracellular matrix. Lobopodia are the most common form of pseudopodia in nature. Filipodia are a type of pseudopodia characterized by the slender, threadlike cytoplasmic projections. They have pointed ends. The pseudopod contains chiefly of ectoplasm. The actin filaments form loose bundles by cross-linking. Filose amoebae members of the subphylum Filosa are examples of amoeba cells that form filopodia.
Reticulopodia are the type of pseudopodia characterized by a reticular network formation of cytoplasmic projections. The pseudopodia form reticulating nets. Examples of organisms forming reticulopodia are the reticulose amoebae of subphylum Endomyxa and foraminiferans of phylum Foraminifera.
These pseudopods are associated with food ingestion more often than locomotion. The pod is coming from the same root word as podiatry, which is referring to the foot. And what I really want you to appreciate, this is used by amoeba either to move around or it could be even used to attack something that it wants to engulf.
And think about what it might take to be able to do this, to be able to grow this type of a pseudo foot, this type of a false foot. You need all sorts of microstructures in here that will extend or contract as necessary. And think about the machinery that you need to do that.
And so the key realization is, sometimes we just imagine cells as these bags of fluid with a few things floating around. But they're these incredibly complex structures, and biologists even today don't fully understand how everything works and they're studying how these things actually come to be.
Now another structure that you'll often see in unicellular organisms that either help them move around or even help move other things around are cilia. So this right over here is a picture of Oxytricha trifallax, which is a unicellular organism. It's a eukaryote. And you can clearly see these projections from its body here, these hairlike structures. Remember this is a unicellular organism.
If we were to, it's actually a fairly, it's a decent sized one. That would be about, something like that would be about 30 micrometers right over there or 30 millionths of a meter or 30 thousandths of a millimeter. So small by our scale, but it's actually pretty big on the scale of it being a cell. And once again, these cilia tend to move in unison to either allow the microorganism to move around or sometimes they're used to move other things around. For examples the cells that line your lungs will have cilia that are used to move things up or down, to move some of the saliva or any particles that are in there.
Now Oxytricha trifallax is particularly interesting as a eukaryote because it doesn't just have one nucleus. It can have two nuclei. Like filopodia, axopodia are long and thin protrusions from the cells.
However, they are more rigid and thus appear needle-like than filopodia which tends to be more flexible in nature. They can be found on the cell surface of various organisms e. In these organisms, axopodium Sin. Here, the microfibrils organize to build the walls of microtubules which then form parallel rows that are joined by links. Based on microscopic studies, these microtubules have been shown to form interlocking double coils with about tubules producing about 5 turns of the coil.
The microtubules make up the core of the axoneme which is the central part of the axopodia run along the entire length of the structure.
Apart from the microtubules, the structure is also composed of cytoplasm which carries such organelles as mitochondria to and from the cytosome. Depending on the organism, these pores vary in size and numbers. Whereas polycystines have been shown to contain many of these pores, phaeodarians contain about three of these pores. The shortening of axopodia has been shown to occur during feeding.
For instance, having captured food, the rapid contraction has been shown to occur due to the breakdown of microtubules. Following the contraction, the axopods then start elongating at the normal rate until they reach the normal length. Some of the main characteristics of axopodia include:. For such organisms as members of the class Actinopoda , axopodia play an important role in feeding. As already mentioned, axopodia have a sticky substance on their surface that is produced by mucocysts.
In addition, they also possess kinetocysts that eject thread-like structures that effectively trap their prey. Using these extrusomes, these organisms are able to trap food material or prey which are then transported to the cell body through the cytoplasmic stream. While smaller prey can be trapped and captured by a single axopodiaum, larger ones are entangled in several axopodia. In some cases, several individuals have been shown to participate in trapping larger prey.
Apart from feeding, axopodia have also been shown to help maintain protists in position in water and even contribute to locomotion. For instance, through a controlled change in length of axopods, a number of heliozoa have been shown effectively transverse in aquatic environments. This, in other organisms, is achieved through the expansion and contraction of ectoplasmic vacuoles located between the axopodia. Here, the organism is able to remain in position or control the direction to which they seek to move.
During cell division , both the axopodia and ectoplasm are lost causing the organism to sink to the bottom. Some of the other notable functions of axopodia include:. Also referred to as rhizopodia or extrathalamous cytoplasm in some books, reticulopodia are thread-like pseudopodia that branch and fuse to form a network that is extremely dynamic.
As is the case with axopodia, reticulopodia are also composed of tubules and the cytoplasm. They can be found in a number of organisms including Endomyxa amoebae and some foraminiferans an ancient group of protists.
In these organisms, reticulopodia are involved in feeding and locomotion. Like axopodia, reticulopodia are also composed of microtubules and cytoplasm. Here, microtubules that make up the pseudopods consist of a unique type of tubulin known as Type 2 beta-tubulin. This tubulin forms helical filaments HFs which is the basis for the microtubule found in foraminiferan reticulopodia. In foraminiferans and other organisms, the retuculopodia extrude through one or more pores apertual openings.
Initially, these pseudopods may be thin and pointed similar to filopodia in appearance. As the amount of cytoplasm in the structure increases, the pseudopodial trunk, known as peduncle, becomes thicker and branches to form new pseudopods. While these pseudopods grow and anastomose link together , they form a network that resembles web-like threads.
Some of the main characteristics of reticulopodia include:. This means that the cytoplasm streams along the lengths of the pseudopods to and from the cell body.
Like axopodia, reticulopodia play an important role in feeding and locomotion.
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