Apicomplexans, a group ofintracellular parasites, havelife 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 anapicomplexan organism is typified by acellular variety with a distinctmorphology andbiochemistry.
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.
Apicomplexans (sporozoans) replicate via ways ofmultiple fission (also known asschizogony). These ways includegametogony,sporogony andmerogony, although the latter is sometimes referred to as schizogony, despite its general meaning.[2]
Merogony is anasexually reproductive process of apicomplexa. After infecting a host cell, a trophozoite (see glossary below) increases in size while repeatedly replicating itsnucleus and otherorganelles.[3] During this process, the organism is known as ameront orschizont.Cytokinesis next subdivides themultinucleated schizont into numerous identical daughter cells called merozoites (see glossary below), which are released into the blood when the host cell ruptures. Organisms whose life cycles rely on this process includeTheileria,Babesia,[4]Plasmodium,[5] andToxoplasma gondii.
Sporogony is a type of sexual and asexual reproduction. It involveskaryogamy, the formation of azygote, which is followed bymeiosis and multiple fission. This results in the production of sporozoites.
Other forms of replication includeendodyogeny andendopolygeny.Endodyogeny is a process ofasexual reproduction, favoured by parasites such asToxoplasma 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 internalbudding.[6]
Asporozoite (ancient Greeksporos, seed +zōon, animal) is the cell form that infects new hosts. InPlasmodium, for instance, the sporozoites are cells that develop in the mosquito's salivary glands, leave the mosquito during a blood meal, and enterliver cells (hepatocytes), where they multiply. Cells infected with sporozoites eventually burst, releasing merozoites into the bloodstream.[8] Sporozoites are motile and they move bygliding.
Amerozoite (G.meros, part [of a series] +zōon, animal) is the result ofmerogony 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. Incoccidiosis, merozoites form the first phase of the internal life cycle of coccidian. In the case ofPlasmodium, merozoites infectred blood cells 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. Beforeschizogony, the merozoite is also known as theschizozoite.[9]
Agametocyte (G.gametēs, partner +kytos, cell) is a name given to a parasite'sgamete-forming cells. A male gametocyte divides to give many flagellatedmicrogametes, whereas the female gametocyte differentiates to amacrogamete.[10]
Anookinete (G.ōon, egg +kinētos, motile) is a fertilisedzygote capable of moving spontaneously. It penetrates epithelial cells lining the midgut ofmosquitoes to form a thick-walled structure known as an oocyst under the mosquito's outer gut lining.[11] Ookinetes are motile and they move bygliding.
Atrophozoite (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.
Ahypnozoite (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 byPlasmodium ovale andP. vivax".[12] Hypnozoites are directly sporozoite-derived.[13]
Abradyzoite (G.bradys, slow +zōon, animal) is a sessile, slow-growing form ofzoonoticmicroorganisms such asToxoplasma 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 abradyzoic merozoite.[14]
Atachyzoite (G.tachys, fast +zōon, animal), contrasting with a bradyzoite, is a form typified by rapid growth and replication. Tachyzoites are the motile forms of thosecoccidians which form tissuepseudocysts, such asToxoplasma andSarcocystis. Typically infecting cellularvacuoles, tachyzoites divide by endodyogeny and endopolygeny. Also known as atachyzoic merozoite (same journal reference as for "bradyzoic merozoite", above).
Anoocyst (G.ōon, egg +kystis, bladder) is a hardy, thick-walled spore, able to survive for lengthy periods outside a host. Thezygote develops within the spore, which acts to protect it during transfer to new hosts. Organisms that create oocysts includeEimeria,Isospora,Cryptosporidium, andToxoplasma.
The dynamics ofgene loss was studied in 41 apicomplexangenomes.[15] Loss of genes employed inamino acid metabolism andsteroid biosynthesis could be explained bymetabolic redundancy with the host.[15] Also,DNA repair genes tend to be lost by apicomplexans with reducedproteome size, probably reflecting a reduced need for DNA repair of genomes with smaller information content.[15] Reduced DNA repair may help explain the elevatedmutation rates in pathogens with reduced genome size.[15]
A merozoite before schizogony, as in the exoerythrocytic phase of the development of thePlasmodium agent after sporozoite invasion of the hepatocyte and before multiple division.