Ultrastructural observations on caecal epithelial cells invaded by first-generation merozoites of Eimeria tenella in vivo.

Transmission electron microscopy was used to study the effects of invasion of caecal epithelial cells by first-generation merozoites of Eimeria tenella. At 53 h post-infection, merozoites were found within mature meronts and in enterocytes of the crypt epithelium. Following merozoite invasion, the terminal web of microfilaments of the invaded cell was disrupted and cell extensions were present on the enterocyte surface. There was a marked morphological alteration of the host cell resulting in loss of microvilli and extensive bulging of cytoplasm into the crypt lumen. Multiple invasion of enterocytes and invasion of goblet cells were observed. Large numbers of mast cells infiltrated the mucosa, identified by the metachromatic staining of their granules, and were found within the lamina propria, epithelium and lumen. Merozoites were found within mast cells and lymphocytes within the lumen.