Cellular structure of a typical, generalised Apicomplexan: 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.

Unusual for a major group of organisms, the apicomplexans are, without exception, exclusively parasitic.^[1] As intracellular parasites, they are exposed to a wide variety of environments during their often complex lifecycles. In order to best meet the requirements of an experienced situation, several different cellular varieties have evolved within each species. Each of these cell types is characterised by its own distinct morphology and biochemistry, and is activated upon reaching a particular step in its lifecycle.

Not all apicomplexa use all the following cell types and division methods and is meant as an outline of a hypothetical generalised apicomplexan organism.

[edit] Methods of asexual replication

Merogony (aka schizogony) is an asexually reproductive process of apicomplexa. After infecting a host cell, a trophozoite increases in size while its repeatedly replicating its nucleus and other organelles.^[2] During this process the organism is known as a schizont. After cytokinesis of the schizont occurs, host cell ruptures, releasing merozoites. Organisms which use this process in their life cycle include Theileria, Babesia,^[3] as well as the malaria parasite Plasmodium.^[4]

Sporogony is a type of sexual or asexual reproduction by multiple fission of a spore or zygote. It is characteristic of a group of parasites caled the sporozoans.

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.^[5]

Endopolygeny is the division into several organisms at once by internal budding.^[5]

[edit] Glossary of cell types

An ookinete, a sporozoite and a merozoite of Plasmodium falciparum.

[edit] Infectious stages

A gametocyte (G. gamete, partner + kytos, hollow) is a name given to a parasite gamete, a cell type produced by meiosis from normal diploid cells. Male gametocytes are called spermatocytes, and female gametocytes are called oocytes.

An ookinete (G. oon, egg + kinetos, motile) is afertilised zygote capable of moving spontaneously. It penetrates epithelial cells lining the midgut of mosquitos to form a thick-walled structure known as an oocyst under the mosquito's outer gut lining.^[6]

A sporozoite (G. sporos, seed + zoon, 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 the liver where they multiply. Cells infected with sporozoites eventually burst, releasing merozoites into the bloodstream.^[7]

A merozoite (G. meros, part [of a series], +zoon, animal) are the result of merogony that takes place within a host cell. In coccidiosis, merozoites form the first phase of the internal life cycle of coccidian. In the case of Plasmodium, merozoites infect red 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-bourne hosts.

[edit] Feeding stages

Lifecycle of the Eimeria parasite

A trophozoite (G. trope, nourishment + zoon, 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 bradyzoite (G. brady, slow + zoon, animal) is a sessile, slow-growing form of zoonotic microorganisms such as Toxoplasma gondii, among others responsible for parasitic infections. In chronic (latent) toxoplasmosis, bradyzoite microscopically presents as clusters enclosed by an irregular crescent-shaped wall (a pseudocyst), in infected muscle and brain tissues.

A tachyzoite (G. tachy, fast + zoon, animal), contrasting with a bradyzoite, is a form typified by rapid growth and replication. Tachyzoites are motile forms of coccidians that form tissue cysts, such as Toxoplasma and Sarcocystis. Typically infecting cellular vacuoles, tachyzoites divide by endodyogeny and endopolygeny.

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

[edit] References