|
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
Schizonts formation involves successive cycles of asexual division that eventually result in the formation of multiple separate "merozoite" forms. Those merozoites are released as the red cell breaks down then go on to infect another red cell. Schizonts therefre look very different depending on which stage of development they represent. 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 below).
Initial asexual replication
The first recognisable stage occurs when the schizonts first divide their chromatin to form two distinct masses. This first stage is the least distinctive and can be difficult to distinguish from a late trophozoite or gametocyte with a double chromatin dot. But often the appearance is clear.
The cartoon image (A) shows the division of chromatin into two distinct purple chromatin masses within the blue parasite cytoplasm (at this point the cytoplams is not divided so indiviual merozoites are not really distinguishable). A clinical image of a parasite at this developmental stage (P.ovale with well shown James'dots) is shown in panel (B).
Immature schizonts
As schizont development proceeds further cycles of division cause the appearance of mutiple separate areas chromatin that will eventually form the merozoies, although at this stage they still lie within a single cytoplasmic mass. The number of divisions varies between species, so in mature schizonts this can contribute to species identification (see schizont gallery). Note that as the parasites develop the haemoglobin is metabolised so the red cell becomes more pale, and the products of red cell breakdown (malaria pigment) become more prominent.
The cartoon image (A) shows the further division of chromatin (Chr) into many discrete massed within the blue parasite cytoplasm (Cy). Indiviual merozoites are still not distinguishable but the malaria pigment is obvious (Pi). A clinical image of a parasite at this developmental stage (again from P.ovale with well shown James'dots and malaria pigment) is shown in panel (B).
Mature schizonts
By this stage the individual merozoites can be distinguished, each with a chromatin dot and cytoplasm; they are now ready for release from the red cell.
The asexual division cycles are now complete cartoon image (A) shows the merozoites (M) as discrete chromatin with blue cytoplasm. Malaria pigment is present (P). The clinical image of a parasite at this developmental stage (again from P.ovale with well shown James'dots and malaria pigment) is shown in panel (B).
Merozoite release
In the final stage the red cell membrane is broken down, swelling then separating to release the merozoites and any malaria pigment into the blood where each merozoite enters a red cell to form a new early trophozoite and increasing the infection load.
Merozoites cause the red cell membrane to be expanded then to break down; the merozoites (M) are now clearly separate and move apart, the pigment (P) is also released during this process (A); this is shown in the clinical image (B) although this brief stage is rarely seen in practice (P.malariae).
| 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.
|
