Extrusome

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Extrusomes are membrane-bound organelles found in eukaryotic cells that are capable of discharging material contained within to the exterior of the cell. Due to the diversity in structure and function, it is unlikely that different types of extrusomes are homologous.

Contents

Some notable extrusomes include mucocysts, which discharge a mucous mass sometimes used in cyst formation, and trichocysts, which discharge a fibrous rod. [1] Nematocysts, the stinging structure found in Cnidarian animals, could be considered extrusomes as well, though those functions are performed by differentiated cells rather than organelles. Other extrusomes include the ancoracyst, a specialized extrusome found in the Provoran eukaryote Ancoracysta twista used to immobilize prey. [2]

Extrusomes and their functions are currently not well understood; many of their supposed functions are in doubt.

Function

In Ciliates

Within the ciliates group, numerous extrusomes–primarily trichocysts–are distributed all across the body. In Paramecium , there was found to be approximately 8000 extrusome structures per cell. Each of these structures is capable of responding to external mechanical or physical stimuli, upon which the structure will rapidly eject a sharp projectile to serve as a defense mechanism. [3] Expulsion of the trichocyst is generated by a change in the structure of proteins within the shaft of the projectile that drastically increases its length.

Predatory ciliates in the classes Prostomatea and Litostomatea have a type of extrusome called a toxicyst. As their name suggests, when extruded, the toxicyst will release toxic material to inhibit the ciliate’s prey. Haptocysts are found in the class Phyllopharyngea and are found at the tips of feeding tentacles. [4] Prey caught by the tentacles will be held in place when the extrusome injects its contents into it, pinning it in place and releasing enzymes to break the prey down.

Mucocysts are found in ciliates and flagellates. They secrete a mucus directly onto the cell surface that often leads to the formation of cysts. This additional layer may serve as a barrier against foreign substances; it may also be re-ingested alongside any organic matter that was captured within. [5]

In Other Groups

Similar to ciliates, most dinoflagellates have trichocysts capable of ejecting a sharp spindle. Though long believed to be genetically dissimilar, recent studies of ribosomal DNA have yielded evidence towards them being more related despite the physiological differences.

Cryptomonads, a group of algae, have a type of extrusomes referred to as ejectosomes. These structures have two connected spiral ribbon-like structures held under tension. When placed under stress, whether mechanical, chemical, or light, the ejectosome will discharge, propelling the protist away from the disturbance. [6]

Cnidocyst, the organelle associated with the stinging cnidocyte cells of jellyfish and other Cnidarians, are another type of extrusome. When the hair-like trigger called the cnidocil receives a chemical or mechanical input, the coiled hollow tube within is ejected, piercing and injecting the target organism with toxin. The time it takes for the entire process to occur is only a few microseconds, and has been measured to reach an acceleration of approximately 40,000 g. [7]

Related Research Articles

<span class="mw-page-title-main">Actinophryid</span> Order of heliozoan protists

The actinophryids are an order of heliozoa, a polyphyletic array of stramenopiles, having a close relationship with pedinellids and Ciliophrys. They are common in fresh water and occasionally found in marine and soil habitats. Actinophryids are unicellular and roughly spherical in shape, with many axopodia that radiate outward from the cell body. Axopodia are a type of pseudopodia that are supported by hundreds of microtubules arranged in interlocking spirals and forming a needle-like internal structure or axoneme. Small granules, extrusomes, that lie under the membrane of the body and axopodia capture flagellates, ciliates and small metazoa that make contact with the arms.

<span class="mw-page-title-main">Cnidocyte</span> Stinging cell used by cnidarians

A cnidocyte is an explosive cell containing one large secretory organelle called a cnidocyst that can deliver a sting to other organisms. The presence of this cell defines the phylum Cnidaria. Cnidae are used to capture prey and as a defense against predators. A cnidocyte fires a structure that contains a toxin within the cnidocyst; this is responsible for the stings delivered by a cnidarian. Cnidocytes are single-use cells that need to be continuously replaced.

<span class="mw-page-title-main">Alveolate</span> Superphylum of protists

The alveolates are a group of protists, considered a major clade and superphylum within Eukarya. They are currently grouped with the stramenopiles and Rhizaria among the protists with tubulocristate mitochondria into the SAR supergroup.

<span class="mw-page-title-main">Cryptomonad</span> Group of algae and colorless flagellates

The cryptomonads are a group of algae, most of which have plastids. They are traditionally considered a division of algae among phycologists, under the name of Cryptophyta. They are common in freshwater, and also occur in marine and brackish habitats. Each cell is around 10–50 μm in size and flattened in shape, with an anterior groove or pocket. At the edge of the pocket there are typically two slightly unequal flagella. Some may exhibit mixotrophy. They are classified as clade Cryptomonada, which is divided into two classes: heterotrophic Goniomonadea and phototrophic Cryptophyceae. The two groups are united under three shared morphological characteristics: presence of a periplast, ejectisomes with secondary scroll, and mitochondrial cristae with flat tubules. Genetic studies as early as 1994 also supported the hypothesis that Goniomonas was sister to Cryptophyceae. A study in 2018 found strong evidence that the common ancestor of Cryptomonada was an autotrophic protist.

<span class="mw-page-title-main">Peniculid</span> Order of single-celled organisms

The peniculids are an order of ciliate protozoa, including the well-known Paramecium and related genera, such as Frontonia, Stokesia, Urocentrum and Lembadion. Most are relatively large, freshwater forms that feed by sweeping smaller organisms into the mouth. They have weird life cycles, and in many cases do not even form resting cysts.

<span class="mw-page-title-main">Litostomatea</span> Class of single-celled organisms

The Litostomatea are a class of ciliates. The group consists of three subclasses: Haptoria, Trichostomatia and Rhynchostomatia. Haptoria includes mostly carnivorous forms such as Didinium, a species of which preys primarily on the ciliate Paramecium. Trichostomatia (trichostomes) are mostly endosymbionts in the digestive tracts of vertebrates. These include the species Balantidium coli, which is the only ciliate parasitic in humans. The group Rhynchostomatia includes two free-living orders previously included among the Haptoria, but now known to be genetically distinct from them, the Dileptida and the Tracheliida.

<i>Paramecium</i> Genus of unicellular ciliates, commonly studied as a representative of the ciliate group

Paramecium is a genus of eukaryotic, unicellular ciliates, widespread in freshwater, brackish, and marine environments. Paramecia are often abundant in stagnant basins and ponds. Because some species are readily cultivated and easily induced to conjugate and divide, they have been widely used in classrooms and laboratories to study biological processes. Paramecium species are commonly studied as model organisms of the ciliate group and have been characterized as the "white rats" of the phylum Ciliophora.

<span class="mw-page-title-main">Unicellular organism</span> Organism that consists of only one cell

A unicellular organism, also known as a single-celled organism, is an organism that consists of a single cell, unlike a multicellular organism that consists of multiple cells. Organisms fall into two general categories: prokaryotic organisms and eukaryotic organisms. Most prokaryotes are unicellular and are classified into bacteria and archaea. Many eukaryotes are multicellular, but some are unicellular such as protozoa, unicellular algae, and unicellular fungi. Unicellular organisms are thought to be the oldest form of life, with early protocells possibly emerging 3.5–4.1 billion years ago.

Tracy Morton Sonneborn was an American biologist. His life's study was ciliated protozoa of the group Paramecium.

<i>Didinium</i> Genus of single-celled organisms

Didinium is a genus of unicellular ciliates with at least ten accepted species. All are free-living carnivores. Most are found in fresh and brackish water, but three marine species are known. Their diet consists largely of Paramecium, although they will also attack and consume other ciliates. Some species, such as D. gargantua, also feeds on non-ciliate protists, including dinoflagellates, cryptomonads, and green algae.

<span class="mw-page-title-main">Trichocyst</span> Organelle

A trichocyst is an organelle found in certain ciliates and dinoflagellates.

<i>Paramecium caudatum</i> Species of single-celled organism

Paramecium caudatum is a species of unicellular protist in the phylum Ciliophora. They can reach 0.33 mm in length and are covered with minute hair-like organelles called cilia. The cilia are used in locomotion and feeding. The species is very common, and widespread in marine, brackish and freshwater environments.

<span class="mw-page-title-main">Protozoa</span> Single-celled eukaryotic organisms

Protozoa are a polyphyletic group of single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic debris. Historically, protozoans were regarded as "one-celled animals".

<span class="mw-page-title-main">Ciliate</span> Taxon of protozoans with hair-like organelles called cilia

The ciliates are a group of alveolates characterized by the presence of hair-like organelles called cilia, which are identical in structure to eukaryotic flagella, but are in general shorter and present in much larger numbers, with a different undulating pattern than flagella. Cilia occur in all members of the group and are variously used in swimming, crawling, attachment, feeding, and sensation.

<span class="mw-page-title-main">Epixenosomes</span> Genus of bacteria

Epixenosomes, also known as Candidatus Epixenosoma, are a genus of bacteria in the phylum Verrucomicrobiota that form a symbiosis with marine ciliates of the genus Euplotidium, where they help to defend their ciliate hosts against predators. It is a monospecific genus, containing only the species Ca. Epixenosoma ejectans.

<i>Chilodonella uncinata</i> Species of single-celled organism

Chilodonella uncinata is a single-celled organism of the ciliate class of alveolates. As a ciliate, C. uncinata has cilia covering its body and a dual nuclear structure, the micronucleus and macronucleus. Unlike some other ciliates, C. uncinata contains millions of minichromosomes in its macronucleus while its micronucleus is estimated to contain 3 chromosomes. Childonella uncinata is the causative agent of Chilodonelloza, a disease that affects the gills and skin of fresh water fish, and may act as a facultative of mosquito larva.

<span class="mw-page-title-main">Warnowiaceae</span> Family of single-celled organisms

The Warnowiaceae are a family of athecate dinoflagellates. Members of the family are known as warnowiids. The family is best known for a light-sensitive subcellular structure known as the ocelloid, a highly complex arrangement of organelles with a structure directly analogous to the eyes of multicellular organisms. The ocelloid has been shown to be composed of multiple types of endosymbionts, namely mitochondria and at least one type of plastid.

<span class="mw-page-title-main">Nematocyst (dinoflagellate)</span> Subcellular structure in unicellular algae

A nematocyst is a subcellular structure or organelle containing extrusive filaments found in two families of athecate dinoflagellates, the Warnowiaceae and Polykrikaceae. It is distinct from the similar subcellular structures found in the cnidocyte cells of cnidarians, a group of multicellular organisms including jellyfish and corals; such structures are also often called nematocysts, and cnidocytes are sometimes referred to as nematocytes. It is unclear whether the relationship between dinoflagellate and cnidarian nematocysts is a case of convergent evolution or common descent, although molecular evidence has been interpreted as supporting an endosymbiotic origin for cnidarian nematocysts.

<i>Polykrikos</i> Genus of single-celled organisms

Polykrikos is one of the genera of family Polykrikaceae that includes athecate pseudocolony-forming dinoflagellates. Polykrikos are characterized by a sophisticated ballistic apparatus, named the nematocyst-taeniocyst complex, which allows species to prey on a variety of organisms. Polykrikos have been found to regulate algal blooms as they feed on toxic dinoflagellates. However, there is also some data available on Polykrikos being toxic to fish.

<span class="mw-page-title-main">Cortical alveolum</span> Cellular organelle found in protists

The cortical alveolum is a cellular organelle consisting of a vesicle located under the cytoplasmic membrane, to which they give support. The term "corticate" comes from an evolutionary hypothesis about the common origin of kingdoms Plantae and Chromista, because both kingdoms have cortical alveoli in at least one phylum. At least three protist lineages exhibit these structures: Telonemia, Alveolata and Glaucophyta.

References

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  3. Harumoto, Terue (1994). "The Role of Trichocyst Discharge and Backward Swimming in Escaping Behavior of Paramecium from Dileptus margaritifer". Journal of Eukaryotic Microbiology. 41 (6): 560–564. doi:10.1111/j.1550-7408.1994.tb01517.x. ISSN   1550-7408.
  4. Taylor, William D.; Sanders, Robert W. (2001), "PROTOZOA", Ecology and Classification of North American Freshwater Invertebrates, Elsevier, pp. 43–96, doi:10.1016/B978-012690647-9/50004-1, ISBN   978-0-12-690647-9 , retrieved 2024-12-04
  5. Wolfe, Jason (1988). "Analysis of Tetrahymena Mucocyst Material with Lectins and Alcian Blue". The Journal of Protozoology. 35 (1): 46–51. doi:10.1111/j.1550-7408.1988.tb04074.x. ISSN   1550-7408.
  6. "X - 10 Cryptophyte With Trichocyst Electron Micrograph". 2011-06-13. Archived from the original on 2011-06-13. Retrieved 2024-12-04.
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