# Apicomplexan life cycle

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Apicomplexa life cycle

Cellular structure of a typical, generalised [apicomplexan](/source/Apicomplexa): 1-polar ring, 2-conoid, 3-micronemes, 4-rhoptries, 5-nucleus, 6-nucleolus, 7-mitochondria, 8-posterior ring, 9-alveoli, 10-golgi apparatus, 11-micropore.

**[Apicomplexans](/source/Apicomplexa)**, a group of [intracellular parasites](/source/Intracellular_parasite), have [life cycle](/source/Biological_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](/source/Apicomplexa) organism is typified by a *cellular variety* with a distinct [morphology](/source/Morphology_(biology)) and [biochemistry](/source/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

See also: [Fission (biology)](/source/Fission_(biology))

Apicomplexans (sporozoans) replicate via ways of [multiple fission](/source/Multiple_fission) (also known as [schizogony](/source/Schizogony)). These ways include **gametogony**, **sporogony** and **merogony**, although the latter is sometimes referred to as schizogony, despite its general meaning.[2]

Merogony is an [asexually reproductive](/source/Asexual_reproduction) process of apicomplexa. After infecting a host cell, a trophozoite ([see glossary below](#Glossary)) increases in size while repeatedly replicating its [nucleus](/source/Cell_nucleus) and other [organelles](/source/Organelles).[3] During this process, the organism is known as a **meront** or **schizont**. [Cytokinesis](/source/Cytokinesis) next subdivides the [multinucleated](/source/Multinucleated) schizont into numerous identical daughter cells called merozoites ([see glossary below](#Glossary)), which are released into the blood when the host cell ruptures. Organisms whose life cycles rely on this process include *[Theileria](/source/Theileria)*, *[Babesia](/source/Babesia)*,[4] *[Plasmodium](/source/Plasmodium)*,[5] and *[Toxoplasma gondii](/source/Toxoplasma_gondii).*

Sporogony is a type of sexual and asexual reproduction. It involves [karyogamy](/source/Karyogamy), the formation of a [zygote](/source/Zygote), which is followed by [meiosis](/source/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](/source/Asexual_reproduction), favoured by parasites such as *[Toxoplasma gondii](/source/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](/source/Budding).[6]

## Glossary of cell types

An ookinete (motile), a sporozoite (motile) and a merozoite (motile) of *[Plasmodium falciparum](/source/Plasmodium_falciparum)*

Two tachyzoites of *[Toxoplasma gondii](/source/Toxoplasma_gondii)*, [transmission electron microscopy](/source/Transmission_electron_microscopy)[7]

### Infectious stages

A **sporozoite** (ancient Greek **sporos**, seed + **zōon**, animal) is the cell form that infects new hosts. In *[Plasmodium](/source/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](/source/Liver) cells ([hepatocytes](/source/Hepatocyte)), where they multiply. Cells infected with sporozoites eventually burst, releasing merozoites into the bloodstream.[8] Sporozoites are motile and they move by [gliding](/source/Gliding_motility).

A **merozoite** (G. **meros**, part [of a series] +**zōon**, animal) is the result of [merogony](/source/Merogony) that takes place within a host cell. During this stage, the parasite infects the host's cells and then replicates its own nucleus and induces cell segmentation in a form of asexual reproduction. In [coccidiosis](/source/Coccidiosis), merozoites form the first phase of the internal life cycle of coccidian. In the case of *[Plasmodium](/source/Plasmodium)*, merozoites infect [red blood cells](/source/Red_blood_cell) and then rapidly reproduce asexually. The red blood cell host is destroyed by this process, which releases many new merozoites that go on to find new blood-borne hosts. Merozoites are motile. Before [schizogony](/source/Schizogony), the merozoite is also known as the **schizozoite**.[9]

A **[gametocyte](/source/Gametocyte)** ([G.](/source/Greek_language) **gametēs**, partner + **kytos**, cell) is a name given to a parasite's [gamete](/source/Gamete)-forming cells. A male gametocyte divides to give many flagellated [microgametes](/source/Microgamete), whereas the female gametocyte differentiates to a [macrogamete](/source/Macrogamete).[10]

An **ookinete** (G. **ōon**, egg + **kinētos**, motile) is a fertilised [zygote](/source/Zygote) capable of moving spontaneously. It penetrates epithelial cells lining the midgut of [mosquitoes](/source/Mosquito) 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](/source/Gliding_motility).

A **[trophozoite](/source/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](/source/Zoonosis) [microorganisms](/source/Microorganism) such as *[Toxoplasma gondii](/source/Toxoplasma_gondii)*, among others responsible for parasitic infections. In chronic (latent) [toxoplasmosis](/source/Toxoplasmosis), bradyzoites microscopically present as clusters enclosed by an irregular crescent-shaped wall ([cysts](/source/Cyst)) 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 [coccidians](/source/Coccidia) which form tissue [pseudocysts](/source/Pseudocyst), such as *[Toxoplasma](/source/Toxoplasma)* and *[Sarcocystis](/source/Sarcocystis)*. Typically infecting cellular [vacuoles](/source/Vacuole), tachyzoites divide by endodyogeny and endopolygeny. Also known as a **tachyzoic merozoite** (same journal reference as for "bradyzoic merozoite", above).

An **oocyst** (G. **ōon**, egg + **kystis**, bladder) is a hardy, thick-walled spore, able to survive for lengthy periods outside a host. The [zygote](/source/Zygote) develops within the spore, which acts to protect it during transfer to new hosts. Organisms that create oocysts include *[Eimeria](/source/Eimeria)*, *[Isospora](/source/Isospora)*, *[Cryptosporidium](/source/Cryptosporidium)*, and *[Toxoplasma](/source/Toxoplasma)*.

		- Life cycle of the *[Babesia](/source/Babesia)* parasite

		- Life cycle of the *[Eimeria](/source/Eimeria)* parasite

		- Life cycle of the *[Toxoplasma](/source/Toxoplasma)* parasite

## Genome size

The dynamics of [gene](/source/Gene) loss was studied in 41 apicomplexan [genomes](/source/Genome).[15] Loss of genes employed in [amino acid](/source/Amino_acid) metabolism and [steroid](/source/Steroid) biosynthesis could be explained by [metabolic](/source/Metabolism) redundancy with the host.[15] Also, [DNA repair](/source/DNA_repair) genes tend to be lost by apicomplexans with reduced [proteome](/source/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](/source/Mutation) rates in pathogens with reduced genome size.[15]

## See also

- [Trematode life cycle stages](/source/Trematode_life_cycle_stages)

## References

1. **[^](#cite_ref-1)** Jadwiga Grabda (1991). *Marine fish parasitology: an outline*. VCH. p. 8. [ISBN](/source/ISBN_(identifier)) [0-89573-823-6](https://en.wikipedia.org/wiki/Special:BookSources/0-89573-823-6).

1. **[^](#cite_ref-tanada_2-0)** Yoshinori Tanada; Harry K. Kaya (1993). [*Insect pathology*](https://books.google.com/books?id=99YwOQnsgGUC&q=difference+between+merogony+and+schizogony&pg=PA415). Gulf Professional Publishing. [ISBN](/source/ISBN_(identifier)) [978-0-12-683255-6](https://en.wikipedia.org/wiki/Special:BookSources/978-0-12-683255-6).

1. **[^](#cite_ref-3)** [*Schizogony definition*](https://web.archive.org/web/20091122102257/http://encarta.msn.com/dictionary_1861702199/schizogony.html). MSN Encarta. Archived from [the original](https://encarta.msn.com/dictionary_1861702199/schizogony.html) on 2009-11-22. Retrieved 2009-12-11.

1. **[^](#cite_ref-4)** Herwaldt; Persing, DH; Précigout, EA; Goff, WL; Mathiesen, DA; Taylor, PW; Eberhard, ML; Gorenflot, AF; et al. (1 April 1996). "A Fatal Case of Babesiosis in Missouri: Identification of Another Piroplasm That Infects Humans". *Annals of Internal Medicine*. **124** (7): 643–650. [doi](/source/Doi_(identifier)):[10.7326/0003-4819-124-7-199604010-00004](https://doi.org/10.7326%2F0003-4819-124-7-199604010-00004). [PMID](/source/PMID_(identifier)) [8607592](https://pubmed.ncbi.nlm.nih.gov/8607592). [S2CID](/source/S2CID_(identifier)) [46733758](https://api.semanticscholar.org/CorpusID:46733758).

1. **[^](#cite_ref-5)** Zhou, M.; Liu, Q.; Wongsrichanalai, C.; Suwonkerd, W.; Panart, K.; Prajakwong, S.; Pensiri, A.; Kimura, M.; Matsuoka, H.; Ferreira, M. U.; Isomura, S.; Kawamoto, F. (April 1998). ["High prevalence of Plasmodium malariae and Plasmodium ovale in malaria patients along the Thai-Myanmar border, as revealed by acridine orange staining and PCR-based diagnoses"](https://doi.org/10.1046%2Fj.1365-3156.1998.00223.x). *Tropical Medicine and International Health*. **3** (4): 304–312. [doi](/source/Doi_(identifier)):[10.1046/j.1365-3156.1998.00223.x](https://doi.org/10.1046%2Fj.1365-3156.1998.00223.x). [PMID](/source/PMID_(identifier)) [9623932](https://pubmed.ncbi.nlm.nih.gov/9623932). [S2CID](/source/S2CID_(identifier)) [23658812](https://api.semanticscholar.org/CorpusID:23658812).

1. ^ [***a***](#cite_ref-smyth_6-0) [***b***](#cite_ref-smyth_6-1) Smyth, James Desmond; Wakelin, Derek (1994). ["*Toxoplasma gondii*"](https://books.google.com/books?id=j1TD9FiEY-sC&pg=PA99). *Introduction to animal parasitology* (3rd ed.). Cambridge University Press. pp. 99–103. [ISBN](/source/ISBN_(identifier)) [0-521-42811-4](https://en.wikipedia.org/wiki/Special:BookSources/0-521-42811-4).

1. **[^](#cite_ref-7)** Rigoulet, Jacques; Hennache, Alain; Lagourette, Pierre; George, Catherine; Longeart, Loïc; Le Net, Jean-Loïc; Dubey, Jitender P. (20 November 2014). ["Toxoplasmosis in a bar-shouldered dove (*Geopelia humeralis*) from the Zoo of Clères, France"](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4236686). *Parasite*. **21**: 62. [doi](/source/Doi_(identifier)):[10.1051/parasite/2014062](https://doi.org/10.1051%2Fparasite%2F2014062). [PMC](/source/PMC_(identifier)) [4236686](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4236686). [PMID](/source/PMID_(identifier)) [25407506](https://pubmed.ncbi.nlm.nih.gov/25407506).

1. **[^](#cite_ref-8)** ["Malaria - Life Cycle Of Plasmodium.swf"](https://web.archive.org/web/20091121224326/http://www.esnips.com/doc/04326a97-13ce-4ec9-b4e5-32c9d7f316d8/Malaria%20-%20Life%20Cycle%20Of%20Plasmodium). esnips. Archived from [the original](http://www.esnips.com/doc/04326a97-13ce-4ec9-b4e5-32c9d7f316d8/Malaria%20-%20Life%20Cycle%20Of%20Plasmodium) on 21 November 2009. Retrieved 2009-12-11.

1. **[^](#cite_ref-9)** ["Schizozoite"](http://medical-dictionary.thefreedictionary.com/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.

1. **[^](#cite_ref-10)** Sinden, RE; Talman, A; Marques, SR; Wass, MN; Sternberg, MJE (August 2010). "The flagellum in malarial parasites". *Current Opinion in Microbiology*. **13** (4): 491–500. [doi](/source/Doi_(identifier)):[10.1016/j.mib.2010.05.016](https://doi.org/10.1016%2Fj.mib.2010.05.016). [PMID](/source/PMID_(identifier)) [20566299](https://pubmed.ncbi.nlm.nih.gov/20566299).

1. **[^](#cite_ref-11)** ["Ookinete (Medical Dictionary)"](http://dictionary.reference.com/browse/ookinete). Dictionary.com. Retrieved 2009-12-11.

1. **[^](#cite_ref-12)** Markus, Miles B. (16 July 2010). "Malaria: Origin of the Term 'Hypnozoite'". *Journal of the History of Biology*. **44** (4): 781–786. [doi](/source/Doi_(identifier)):[10.1007/s10739-010-9239-3](https://doi.org/10.1007%2Fs10739-010-9239-3). [PMID](/source/PMID_(identifier)) [20665090](https://pubmed.ncbi.nlm.nih.gov/20665090). [S2CID](/source/S2CID_(identifier)) [1727294](https://api.semanticscholar.org/CorpusID:1727294).

1. **[^](#cite_ref-13)** Markus, Miles B. (22 March 2018). "Biological concepts in recurrent *Plasmodium vivax* malaria". *Parasitology*. **145** (13): 1765–1771. [doi](/source/Doi_(identifier)):[10.1017/S003118201800032X](https://doi.org/10.1017%2FS003118201800032X). [PMID](/source/PMID_(identifier)) [29564998](https://pubmed.ncbi.nlm.nih.gov/29564998). [S2CID](/source/S2CID_(identifier)) [206250162](https://api.semanticscholar.org/CorpusID:206250162).

1. **[^](#cite_ref-14)** Markus, M. B. (15 November 2016). "Terms for coccidian merozoites". *Annals of Tropical Medicine & Parasitology*. **81** (4): 463. [doi](/source/Doi_(identifier)):[10.1080/00034983.1987.11812147](https://doi.org/10.1080%2F00034983.1987.11812147). [PMID](/source/PMID_(identifier)) [3446034](https://pubmed.ncbi.nlm.nih.gov/3446034).

1. ^ [***a***](#cite_ref-:0_15-0) [***b***](#cite_ref-:0_15-1) [***c***](#cite_ref-:0_15-2) [***d***](#cite_ref-:0_15-3) Derilus, D.; Rahman, M.Z.; Serrano, A.E.; Massey, S.E. (January 2021). ["Proteome size reduction in Apicomplexans is linked with loss of DNA repair and host redundant pathways"](https://linkinghub.elsevier.com/retrieve/pii/S1567134820304731). *Infection, Genetics and Evolution*. **87** 104642. [doi](/source/Doi_(identifier)):[10.1016/j.meegid.2020.104642](https://doi.org/10.1016%2Fj.meegid.2020.104642). [PMC](/source/PMC_(identifier)) [7936648](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7936648). [PMID](/source/PMID_(identifier)) [33296723](https://pubmed.ncbi.nlm.nih.gov/33296723).

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