Fine Structure of First-Generation Merozoites of Eimeria bovis

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Abstract
The fusiform merozoite is enclosed by a cell membrane. Another membrane underlying the cell membrane encloses the cytoplasm except at the anterior end of the merozoite where this iner membrane terminates forming the polar ring. Approximately 22 subpellicular fibrils extend posteriorly from the polar ring. A conoid, consisting of 1 or more fibrils wound in a tight helix, is situated within the polar ring. A paired organelle extends posteriorly through the conoid from the anterior end. Each member of the paired organelle is club-shaped having a narrow neck within the conoid region and a wider posterior portion. A median rod parallels the necks of the paired organelle. The region of the merozoite between the conoid and the ovoid glycogen bodies is tightly packed with many tortuous structures having indistinct borders. Numerous ribosomes as well as one or two mitochondria are scattered among these structures. A dense, membrane-enclosed body, possibly a lysosome, is occasionally seen near the mitochondria. The Golgi apparatus lies next to the flattened anterior edge of the nucleus. In some specimens a punctate invagination of the cell surface was seen near the level of the Golgi apparatus. Several cisternae of rough-surfaced endoplasmic reticulum are found both anterior and posterior to the nucleus. The merozoites lie free in a vacuole of the host cell. Blebbing of the host cell''s vacuolar membrane releases vesicles into the vacuole. The outer surface of the host cell has numerous microvilli. A fine, fibrous layer exists in the host cell cytoplasm surrounding the vacuole.

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