An ultrastructural study of the asexual development of a presumed Isospora sp. in mononuclear, phagocytic cells of the evening grosbeak (Hesperiphona vespertina).

Merozoites of a presumed Isospora species were invested by a close-fitting vacuole of host-cell origin following their entry into mononuclear, phagocytic cells of the evening grosbeak (Hesperiphona vespertina). Here, the merozoites transformed into trophozoites that were bound by a plasmalemma, beneath which lay short segments of the inner membrane complex from the pellicle of the former merozoite. Nuclear division followed and in schizonts of various sizes (probably representing several asexual generations), merozoite buds appeared around the peripheral cytoplasm. Merozoites also were seen forming within the internal cytoplasm of some schizonts. A conical, anterior projection of the nucleus containing microtubules (the centrocone) and an adjacent pair of centrioles often were seen in forming merozoites. Some maturing merozoites lay singly within distended parasitophorous vacuoles in the cytoplasm of the phagocytic cells, whereas in other vacuoles, groups of merozoites were seen. In addition to the well-documented "standard" apicomplexan features, merozoites of the Isospora sp. contained two spheroidal vesicles closely invested by several layers of membrane, which lay anterior to the nucleus near a Golgi complex. The merozoites possessed about 26 pellicular microtubules, three polar rings, two or three rhoptries, and two or more micropores. Two types of merozoites were seen; one, electron-dense and the second, considerably less so. In certain of their ultrastructural features, the merozoites of the Isospora sp. in the grosbeak resemble more closely the bradyzoites of many of the heteroxenous, isosporoid species than the merozoites of the conventional, gut-based species of Isospora and Eimeria.