Secretion of multilamellar whorls by Eimeria tenella zoites

Multilamellar whorls were demonstrated by transmission electron microscopy to be associated with sporozoites and all generations of merozoites of Eimeria tenella, in chicken cecal tissue fixed without tannic acid or ruthenium red at room temperature. Whorls were found within the parasitophorous vacuoles of recently invaded cells at all stages of development, suggesting a role in the formation of the host parasite interface. Whorls were also associated with intraluminal third-generation merozoites prior to host cell invasion and appeared to be secreted directly through the pellicle. Membranous sheaths, shown by serial sectioning to be derived from intracellular whorl material, were observed enveloping some intraluminal merozoites. In many third-generation merozoites, whorl material was located within discrete novel organelles (here termed lamellosomes) located in the apical region. These densely staining spherical organelles were morphologically distinct from micronemes and rhoptries and were one-third the size of dense granules. These findings confirm that whorls are nonartifactual secretions whose lamellar organization is lost during normal fixation on ice without tannic acid. It is hypothesized that whorls secreted prior to invasion are involved in protection of the motile zoite, immune evasion, or some aspect of gliding motility.