Also known as pseudopodia (singular noun: pseudopodium), pseudopods are temporary extensions of the cytoplasm (also referred to as false feet) used for locomotion and feeling. They can be found in all sarcodines as well as a number of flagellate protozoa that either exist as parasites or as free living organisms.
In higher animals, pseudopods can be observed in a number of leukocytes (phagocytic cells) that use the structure to trap and destroy invading microbes. Depending on the type of cell, there are four main types that not only vary in appearance (and general morphology), but also have different functions.
For instance, in some organisms, pseudopods contain microtubules that significantly contribute to cell movement.
Filopodia are slender actin-based structures that serve sensory and locomotory functions. Like other pseudopods, filopodia are cellular protrusions and thus extend from the cell surface. However, compared to pseudopods found in single-celled organisms, filopodia are mostly found in some cells of multicellular organisms where they extend into the extracellular matrix and are involved in signaling.
* Some single-celled organisms such as members of the genus Dictyostelium use filopodia for feeding.
The formation of filopodia starts with the nucleation of actin filaments under the influence of nucleators (a group of proteins). Although two models have been proposed to explain filopodia initiation (induction), the process appears to be triggered by the binding of GTPase Cdc42 to an essential regulator known as N-WASP.
This results in the activation of N-WASP which in turn binds to Profilin and Arp2/3 to form a complex that nucleates the formation of a new pseudopod.
Although two models of filopodia initiation have been proposed , these being convergent elongation model and the tip nucleation model, further studies have shown that they are not mutually exclusive.
According to these studies, the two models can actually co-exist particularly when considering the diverse and variable nature of these structures.
With regards to structure, filopodia are slender, cylindrical protrusions that range between 100 and 200nm in diameter and 10um in length. However, some of the filopodia on the cell surface are extremely short barely protruding from the cell surface. These filopodia are known as microspikes.
The actin filaments (10 to 30) make up the central core of the structure. Here, the filaments are tightly packed together in a parallel manner to form the shaft of the pseudopod.
Within the filopodia, the filaments overlap and are aligned in uniform polarity; where the barbed end of the filament is oriented towards the filopodial tip of the structure. The barbed wind up in the tip complex that consists of actin-binding proteins and filaments. At the base region of the pseudopod, the filaments run into the actin web located beneath the cell membrane.
While the addition of actin monomers to the actin filaments occur at the tip of the structure as it extends in length, the filaments have been shown to continually cycle backward towards the base through a retrograde flow which is myosin-dependent. It's the rate between this flow and monomer addition at the top that influences the overall rate of growth.
Based on electron tomography studies, findings have shown this growth to be at a rate of about 0.2um/s (which can increase to about 25um a minute) before the structure reaches the critical length. At this point, filopodia will either produce new actin structures or start retracting.
Having reached the critical length, some filopodia have been shown to bind and fuse with the plasma membrane producing actin bundles.
* Filopodia are flexible enough to wave in the extracellular matrix. However, they are also sufficiently strong which allows the structural integrity to be maintained even when the structure extends to over 30um in length.
Some of the most common types of filopodia include:
· Cytonemes - Found in the wings of Drosophila species, this type of filopodia can grow to over 800um where they are involved in cell to cell communication
· Myopodia - A type of filopodia that can be found on the cell surface of muscle cells where they intermingle with other types of filopodia.
In multicellular organisms, filopodia play a number of physiological functions including wound healing, cell signaling as well as cell development. Given that filopodia extend into the extracellular matrix, they are able to sense chemicals in their surrounding which in turn allows the cell to respond appropriately.
Here, receptors within filopodia receive chemical information in the extracellular matrix which is then transmitted downwards to the cell (through a signaling pathway).
In the extracellular matrix, filopodia may identify targets required for adhesion which allows for the generation of guidance cues as well as traction forces that ultimately contributes to the movement of the cell. Through this process, cells are capable of such activities as axonal path finding and adhesion to epithelial cells that contribute to cell migration.
* Filopodia also contribute to adhesion zippering, a process where they participate in cell alignment and adhesion that reduces the gap between cells.
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.g. members of the phylum Antinopoda) where they are involved in feeding and locomotion.
In these organisms, axopodium (Sin. axopodium) originate from axoplasts (a membrane-bound cavity near the nucleus that consists of microfibrillar material and granulum). 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 500 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.
* In a cell, axopodia emerge from axoplasts through pores located on the capsule wall. 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:
· Rigid - As such, they are more resistant to bending compared to the other pseudopods
· Thin and elongated - Needle-like
· During unfavorable conditions, they can retract (rapid contraction) where the filaments are reabsorbed following the dispersion of fibrils in the axoplasm
· Axopodia microtubules have a diameter of about 220 A
· Axopodia are also lost during cell division
· Have a sticky surface which is attributed to the presence of extrusomes
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.
Support and Locomotion
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:
· Distinct from the others in that they are highly branched and form anastomosed networks
· Depending on the organism, reticulopodia can extend a few centimeters from the cell body of the organism
· Can rapidly extend and retract at a rate of about 20um/s
· In the absence of the cytoskeletal microtubules that provide structural support, reticulopodia transform into a series of droplets through a process known as beading response
· Within the pseudopods, the transport of particles is bidirectional. 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. However, their primary function is in food gathering and feeding. During feeding, the organism spreads the pseudopods (which looks like an irregular web) which scour their immediate surfaces and gather available food material to be ingested.
Prey may include such single-celled organisms as bacteria that are trapped in the web and taken into the food vacuoles for digestion. Apart from feeding, reticulopodia are also used for locomotion. However, this is not their primary function.
Lopodium is the most common type found in such organisms as Amoeba proteus. Lobopods are characterized by finger-like tubular pseudopodia consisting of ecto and endoplasm. However, they have also been shown to contain actin and myosin (microfilaments) which contribute to the overall movement.
Unlike the other pseudopods, microtubules in lobopodia are poorly developed. In many amoebas, lobopodia are primarily involved in locomotion.
Formation and Locomotion
In such organisms, the formation of lobopodia is influenced by chemical signals in their environment. In the presence of a food substance, chemical signals influence the direction to which the amoeba moves. Here, the molecules (from the food material) bind to the receptors located on the organism's cell membrane which stimulates the formation of filaments through the aggregation of globular actin.
With the addition of globular actin, the structure (filament) continues to elongate which in turn causes a protrusion of the membrane (this action results in pseudopodia formation). The protruding lobopods are them filled with cytoplasm as it extends. In the event that the molecules disappear, globular actin disaggregate which stops the pseudopods from elongating further.
If the molecules persist, myosin, which acts as the motor proteins, interacts with the actin to push the cell body in the direction of the pseudopod.
* Myosin activity (as motors) requires energy (ATP).
* The viscosity of cytoplasm has also been shown to change as it flows in and out of the pseudopods.
During feeding, lobopodia also surround food material and engulf them within a vesicle where they are acted upon by various enzymes. Waste products are then excreted through vacuoles that open up to the environment.
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