Ultrastructure of two types of first-generation merozoites of Eimeria bovis.

Eimeria bovis has two types of first-generation merozoites with distinct ultrastructural characteristics. Type I merozoites were relatively large (means = 13.2 x 1.5 micron) and crescent-shaped, contained numerous micronemes and amylopectin granules, had a posteriorly located nucleus and a conical-shaped posterior tip, and were highly motile and capable of penetrating cultured cells. Type II merozoites were small (means = 5.9 x 0.9 micron) and spindle-shaped, had a centrally-located nucleus, few micronemes, few or no amylopectin granules, a dome-shaped posterior tip and little motility, and appeared to be incapable of penetrating cultured cells. It is possible that these two types of merozoites have considerably different roles in the life cycle of E. bovis.