Transmission electron microscopy of meront development of Eimeria vermiformis Ernst, Chobotar and Hammond, 1971 (Apicomplexa, Eucoccidiorida) in the mouse, Mus musculus.

First and second generation meronts of Eimeria vermiformis developed in epithelial cells of the crypts of Lieberkühn. They were usually between the host cell nucleus and the basement membrane. Sporozoite organelles dedifferentiated with the first generation meront's development except for the refractile body and the apical complex, which persisted. After several nuclear divisions, the apical complex dedifferentiated further until only micronemes remained attached by a duct system to the plasmalemma. The form of the apical complex was highly variable. Sometimes the duct system was absent and the micronemes were attached directly to the plasmalemma or a dense material on it. Crescent body-like material was often present in the parasitophorous vacuole next to the microneme structure. The microneme structure was not present in second generation meronts but evaginations of the plasmalemma, cytoplasmic outpocketings, and cytoplasmic vesicles were associated with the round granular bodies in the parasitophorous vacuoles. During first generation merogenesis, invaginations from the parasitophorous vacuole formed channels into the meront along which merozoites budded. Micropores were often at the ends of these invaginations. These and other micropores of the meront had a dense U-shaped band for a collar while those of the merozoites had a collar with a double band of dense material that connected to the inner membrane. First generation merozoites budded randomly from the meront, resulting in a residual body that was usually in the middle of the parasitophorous vacuole. Second generation merozoites budded in one direction, resulting in a peripheral residual body and merozoites that were parallel in an oblong parasitophorous vacuole.