Apicomplexan life cycle
Apicomplexans, a group of intracellular parasites, have life cycle stages that allow them to survive the wide variety of environments they are exposed to during their complex life cycle.[1] Each stage in the life cycle of an apicomplexan organism is typified by a cellular variety with a distinct morphology and biochemistry.
Not all apicomplexa develop all the following cellular varieties and division methods. This presentation is intended as an outline of a hypothetical generalised apicomplexan organism.
Methods of asexual replication
Apicomplexans (sporozoans) replicate via ways of
Merogony is an
Sporogony is a type of sexual and asexual reproduction. It involves karyogamy, the formation of a zygote, which is followed by meiosis and multiple fission. This results in the production of sporozoites.
Other forms of replication include endodyogeny and endopolygeny. Endodyogeny is a process of asexual reproduction, favoured by parasites such as Toxoplasma gondii. It involves an unusual process in which two daughter cells are produced inside a mother cell, which is then consumed by the offspring prior to their separation.[6]
Endopolygeny is the division into several organisms at once by internal budding.[6]
Glossary of cell types
Infectious stages
A sporozoite (ancient Greek sporos, seed + zōon, animal) is the cell form that infects new hosts. In Plasmodium, for instance, the sporozoites are cells that develop in the mosquito's salivary glands, leave the mosquito during a blood meal, and enter liver cells (hepatocytes), where they multiply. Cells infected with sporozoites eventually burst, releasing merozoites into the bloodstream.[8] Sporozoites are motile and they move by gliding.
A merozoite (G. meros, part [of a series] +zōon, animal) is the result of
A gametocyte (G. gametēs, partner + kytos, cell) is a name given to a parasite's gamete-forming cells. A male gametocyte divides to give many flagellated microgametes, whereas the female gametocyte differentiates to a macrogamete.[10]
An ookinete (G. ōon, egg + kinētos, motile) is a fertilised zygote capable of moving spontaneously. It penetrates epithelial cells lining the midgut of mosquitoes to form a thick-walled structure known as an oocyst under the mosquito's outer gut lining.[11] Ookinetes are motile and they move by gliding.
A trophozoite (G. trophē, nourishment + zōon, animal) is the activated, intracellular feeding stage in the apicomplexan life cycle. After gorging itself on its host, the trophozoite undergoes schizogony and develops into a schizont, later releasing merozoites.
A hypnozoite (G. hypnos, sleep + zōon, animal) is a quiescent parasite stage that is best known for its "... probable association with latency and relapse in human malarial infections caused by Plasmodium ovale and P. vivax".[12] Hypnozoites are directly sporozoite-derived.[13]
A bradyzoite (G. bradys, slow + zōon, animal) is a sessile, slow-growing form of zoonotic microorganisms such as Toxoplasma gondii, among others responsible for parasitic infections. In chronic (latent) toxoplasmosis, bradyzoites microscopically present as clusters enclosed by an irregular crescent-shaped wall (cysts) in infected muscle and brain tissues. Also known as a bradyzoic merozoite.[14]
A tachyzoite (G. tachys, fast + zōon, animal), contrasting with a bradyzoite, is a form typified by rapid growth and replication. Tachyzoites are the motile forms of those
An oocyst (G. ōon, egg + kystis, bladder) is a hardy, thick-walled spore, able to survive for lengthy periods outside a host. The
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Life cycle of the Babesia parasite
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Life cycle of the Eimeria parasite
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Life cycle of theToxoplasmaparasite
Genome size
The dynamics of gene loss was studied in 41 apicomplexan genomes.[15] Loss of genes employed in amino acid metabolism and steroid biosynthesis could be explained by metabolic redundancy with the host.[15] Also, DNA repair genes tend to be lost by apicomplexans with reduced proteome size, probably reflecting a reduced need for DNA repair of genomes with smaller information content.[15] Reduced DNA repair may help explain the elevated mutation rates in pathogens with reduced genome size.[15]
See also
References
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- ^ Schizogony definition. MSN Encarta. Archived from the original on 2009-11-22. Retrieved 2009-12-11.
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- ^ "Malaria - Life Cycle Of Plasmodium.swf". esnips. Archived from the original on 21 November 2009. Retrieved 2009-12-11.
- ^ "Schizozoite", Farlex Partner Medical Dictionary, 2012,
A merozoite before schizogony, as in the exoerythrocytic phase of the development of the Plasmodium agent after sporozoite invasion of the hepatocyte and before multiple division.
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- ^ "Ookinete (Medical Dictionary)". Dictionary.com. Retrieved 2009-12-11.
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