Ultrastructure of schizonts in the liver of cats with experimentally induced cytauxzoonosis.

Schizonts in the liver of 2 cats with cytauxzoonosis were studied by both light and electron microscopies. By light microscopy, the cytoplasm of macrophages in the sinusoids and small vascular channels contained schizonts with cytomeres or both cytomeres and mature merozoites. By electron microscopy, it was determined that schizogony occurred in 4 stages. The earliest stage was the presence of a multilobed structure containing finely granular protoplasm in the cytoplasm of the macrophage. The 2nd stage was an increase in height and number of the lobulations on the surface of the schizont. The 3rd stage involved the development of cytomeres and the appearance of a polar ring and rhoptries in everted sacculations on the cytomere membrane. Nuclei and mitochondria were incorporated into the sacculations before the release of mature merozoites into the host cell cytoplasm. In the last stage of schizogony, following massive merozoite formation and reduction in size of the schizont, residual nuclei divided by multiple fission. Each nuclear division became incorporated into a developing merozoite having preformed rhoptries, mitochondria, and a polar ring.