Fine structure of the erythrocytic stages of Plasmodium vivax and the host cell alterations.

Fine structural analysis of the trophic and sexual stages of Plasmodium vivax obtained from naturally infected humans revealed that in general, the structural features as well as certain specialized functions such as haemoglobin ingestion and utilization etc., are similar to those described for other malaria parasites. Young trophozoite is characterised by less condense cytoplasm with scattering of free ribosomes and minimum amount of endoplasmic reticulum. The trophozoite grows by feeding on the host cell cytoplasm with its cytostome. Older trophozoite gets considerably enlarged and its cytoplasm appears coarsely granular due to an increase in endoplasmic reticulum and ribosomal content. The two sexual forms, the macro- and microgametocyte, can be distinguished on the basis of their fine structure. The macrogametocyte has high ribosomal density and more osmiophilic bodies than microgametocyte and possesses a compact dense nucleus with nucleolus-like region whereas the microgametocyte has large diffuse nucleus without nucleolus. Caveola-vesicle complexes and cytoplasmic clefts are observed in all erythrocytes infected with P. vivax and the incidence of these host cell alterations increases as the intraerythrocytic parasite matures.