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.