Ultrastructure of rhoptry development in Plasmodium falciparum erythrocytic schizonts

Prior to the separation of merozoites from the Plasmodium falciparum schizo nt, various stage-specific organelles are synthesized and assembled within each merozoite bud. The apical ends of the merozoites are initiated close t o the ends of endomitotic spindles. At each of these sites, the nuclear mem brane forms coated vesicles, and a single discoidal or cuplike Golgi cister na appears. Reconstruction from serial sections indicates that this structu re receives vesicles from the nuclear envelope and in turn gives off coated vesicles to generate the apical secretory organelles. Rhoptries first form as spheroidal structures and grow by progressive fusion of small vesicles around their margins. As each rhoptry develops, 2 distinctive regions separ ate within it, an apical reticular zone with electron-lucent areas separate d by cords of granular material, and a more homogenously granular basal reg ion. The apical part elongates into the duct, with evidence for further ves icular fusion at the duct apex. The rounded rhoptry base becomes progressiv ely more densely packed to form a spheroidal mass, and compaction also occu rs in the duct. Typically, one rhoptry matures before the other. Cryofractu red rhoptry membranes show asymmetry in the sizes and numbers of intramembr anous particles at the internally- and externally-directed fracture faces.