Biology of the schizont

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Biology of the Schizont

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After the completion of the trophozoite stage the pathways diverge, with a proportion of cells entering sexual development (to form gametocytes), while others enter asexual replication as schizonts. The asexual stage begins with the first cycle of divsion forming a recognisable “schizont” with separate chromatin masses. Then concludes when the individual “merozoites” are released to infect new erythrocytes forming new trophozoites. Formation and release of merozoites (1) The stage begins with the first cycle of asexual division producing two chromatin masses (2) This is followed by further cycles of replication (3) In this case this results in the formation of 8 daughter parasites (4) The daughter parasites mature and the red cell ruptures to release the “merozoites” (5) The released merozoites very rapidly infect new red cells (so rapid that free merozoites will not usually be seen in blood). Morphological features and relevance (1) The number of replication cycles differs between species: the typical number of merozoites formed differs between species with as few as 8 (in P.malariae) up to a possible 32 (in P.vivax) (2) This stage may not always be seen in blood: the schizonts of P.falciparum adhere within the small vessels so is not seen in blood unless infection is very severe Mature schizont releasing merozoites The progressive maturation of this parasite stage means that they have a wide range of morphological forms. However, these can be readily recognised on blood films by reference to their biology. [ See clinical images illustrating schizont development ] Relevance of schizonts to clinical biology The release of merozoites from schizonts exposes the body to large amounts of free parasite antigens no longer contained within the erythrocytes - the result is an immune response causing high fever and illness symptoms. In some cases the development of parasites is synchronous so that all schizonts mature and release their merozoites at the same time - although rarely seen now, this pattern of development may produce a pattern of remitting fever with a distinct periodicity depending on species: underlying the older descriptive terms tertian or quartan malaria.