Rhoptries

From Greek: rhopalon = club. This term describes clubbed cell organelles (mostly two) at the apical pole of motile stages (merozoites, sporozoites) of Apicomplexa, which are active during the process of penetration into their host cell. These bulbous organelles (first called paired organelles) start immediately at the inner side of the cell membrane and stretch into the interior of the first third of sporozoites, merozoites, endozoites, cystozoites, etc. Their apical pole (neck) has a small diameter, while their terminal portion (bulb) is swollen (e.g., in Eimeria species or Toxoplasma gondii) and in some species nearly spherical in diameter (e.g., in Plasmodium species (Figs. 1 and 2). In species with an apical conoid (e.g., Eimeria, Toxoplasma, Sarcocystis, etc.), the apical portions of the rhoptries pass the protrudable conoid (Fig. 1), while in the case of the conoid-less Plasmodium species the neck of the rhoptries is found inside the opening of the apical polar ring, which is...