Introduction
The phylum Apicomplexa holds some of the most important human parasites (Plasmodium, Toxoplasma, Cryptosporidium). This topic covers the phylum's general features and Toxoplasma gondii: the cat is its definitive host; every warm-blooded animal, including about a third of all people, can be an intermediate host. It matters most in pregnancy (congenital toxoplasmosis) and in immunodeficiency (toxoplasmic encephalitis).
Phylum Apicomplexa: general features
- All are obligate intracellular parasites of animals; there are no free-living species.
- No organelles of locomotion in the invasive stages (only the male gametes of some species have flagella). The invasive "zoites" move by gliding, pulled forward by an actin–myosin motor under the pellicle.
- The apical complex, which gives the phylum its name, is a set of structures at the front end used to invade host cells:
- polar rings and (in Coccidia) a hollow cone of microtubules, the conoid, which pushes out during invasion;
- secretory organelles: micronemes (adhesion proteins), rhoptries (club-shaped; proteins that build the vacuole and take over the host cell) and dense granules (they modify the vacuole after entry).
- The parasite enters actively, forming a parasitophorous vacuole from the host cell membrane. This vacuole does not fuse with lysosomes, so the parasite is protected while it multiplies. Host organelles (mitochondria, endoplasmic reticulum) gather around the vacuole and feed it.
- Apicoplast: a remnant plastid (from an ancient algal endosymbiont) without photosynthesis; it makes fatty acids and other molecules and is a drug target (clindamycin, doxycycline act on it).
- Alternation of generations with three phases of reproduction:
- Schizogony (merogony): asexual multiple fission (in Toxoplasma also endodyogeny, two daughters forming inside the mother cell);
- Gametogony: formation of male (micro-) and female (macro-) gametes, which fuse into a zygote;
- Sporogony: the zygote (often inside an oocyst) divides to form sporozoites, the infective stage.
- The life cycle is haploid: the zygote is the only diploid stage, and meiosis happens right after fertilisation.