Falciparum malaria in naturally infected human patients: VIII. Fine structure of intraerythrocytic asexual forms before and during chloroquine treatment.

The fine structure of the intraerythrocytic asexual forms of Plasmodium falciparum is described before and during treatment with chloroquine (CQ) in the venous blood of 27 Saudi patients whe were heavily infected with the parasite. Before treatment, the "pear" shaped merozoite was rapidly transformed into a ring-shaped trophozoite. This was enclosed in a parasitophorous vacuole bounded by a single membrane derived from the host erythrocyte. The parasite feeding mechanism involves several previously undescribed endocytic organelles that function to internalize the erythrocyte haemoglobin into the central food vacuole (FV). Crystalline pigment particles, the trophozoite final metabolic product, accumulated in residual membrane-bound vacuoles. Fully mature schizonts releasing newly produced merozoites were seen for the first time at the electron microscopy level. During treatment, the parasite cytoplasm appeared divided into a vacuolated, electron-dense, ribosomal region and a less dense FV. The feeding mechanism was apparently inhibited; this was indicated by severe damage to the FV and absence of the endocytic organelles, seen in the healthy parasite, causing its starvation and death.