Fine Structural Aspects of Nuclear Division and Merogony of Eimeria ninakohlyakimovae in Cultured Cells

Fine structural aspects of nuclear division and development of first-generation merozoites of Eimeria ninakohlyakimovae in cultures of ovine trachea, kidney, and thyroid cells fixed 12, 15, and 21 days after inoculation of sporozoites were studied. In some apparently dividing nuclei, eccentric or centrally located intranuclear spindles, with centrioles near their poles, were observed. Microtubules resembling astral rays were observed around some centrioles. In some schizonts, numerous invaginations or infoldings of the limiting membrane occurred, which sometimes led to the isolation of spheroidal blastophores, each having a number of nuclei and limited by a surface membrane derived from that of the schizont. In large schizonts and developing merozoites, aggregations of microtubules were seen. The first indication of merozoite formation was the appearance, just interior to the schizont membrane, of relatively electron-dense thickenings, each of which apparently represented the beginning of the inner membrane complex of the pellicle of a merozoite. Later, a conoid, micronemes, rhoptry anlagen, and subpellicular microtubules appeared in association with each thickening. A nucleus was usually seen nearby. Sites of merozoite formation were always observed in close proximity with a membrane surface of schizonts, infoldings of schizonts, or blastophores. Merozoite development proceeded by elongation of the merozoite and incorporation of certain organelles, such as the nucleus, Golgi apparatus, and mitochondria. Completely formed merozoites were separated posteriorly from the residual body by a pinching-off process. Some merozoites apparently undergoing a kind of binary fission were observed.