Fine structural aspects of an embryo-adapted strain of Eimeria tanella in the epithelial cells of the chorio-allantoic membrane.

The fine structure and development of the endogenous stages of an embryo-adapted strain of Eimeria tenella in chicken embryos is described. First-generation merozoites were formed by endodyogeny in uninucleate developmental stages as well as schizogony and the endoplasmic reticulum of merozoites of the second-generation schizonts appeared as a circular structure, often enclosing other organelles. In contrast to the parent strain, development of the second-generation schizonts was restricted to cells of epithelial origin. These observations confirm that the complete endogenous cycle of the embryo-adapted strain of E. tenella is restricted to epithelial cells of the chorio-allantoic membrane (CAM) and that no major ultrastructural changes have occurred as a result of repeated embryo passage.