Three-dimensional ultrastructure of the ring stage of Plasmodium falciparum: evidence for export pathways.

The three-dimensional structure of the Plasmodium falciparum ring stage has been explored by reconstruction from serial sections and stereoscopic examination of tilted sections. The ring-like light microscopic appearance is related to the shape and contents of the biconcave discoidal parasite at this stage, its thick perimeter containing most of the ribosomes and its thin center containing smooth membrane organelles. The shapes of rings vary between flat and curved cuplike forms. The rough endoplasmic reticulum is a branched network continuous with the nuclear envelope. Evidence for a simple Golgi complex is seen in the presence on the outer nuclear envelope of a locus of coated vesicle budding associated with a single membranous cisterna or cluster of smooth vesicles. In middle and late stage rings this complex migrates along an extension of the nuclear envelope continuous with the rough endoplasmic reticulum. Evidence is also presented for a mechanism of exporting membrane from the parasite into the parasitophorous vacuole membrane and beyond into the red blood cell, by means of double-membraned vesicle-based exocytosis.