Ultrastructural observations of development of Eimeria tenella in a novel established avian-derived cell line

Transmission electron microscopy was used to study the in vitro development of Eimeria tenella in a novel established avian-derived cell line (designated CEV-1/F7) used for antigen production in chicken immunization studies. Sporozoites of E. tenella were inoculated onto cell monolayers and the cells were fixed at 24-h intervals. Large numbers of intracellular sporozoites were seen at 24 h postinoculation (p.i.), and trophozoites were identified at 24-48 h p.i. Immature schizonts, some with budding merozoites, were seen by 48 h p.i. At 72-96 h p.i., immature and mature schizonts and extracellular merozoites were observed. No merozoite invasion occurred, but immature second-generation schizogony was seen in parasitophorous vacuoles of first-generation schizonts. No further development occurred and degeneration of most schizonts was seen by 120-144 h p.i. The results confirmed synchronous development of E. tenella until 48 h p.i., followed by asynchronous development and ultrastructural degeneration with increased incubation time.