The fine structure of Leucocytozoon simondi. II. Megaloschizogony.

Abstract

Incompletely divided cytomeres released from hepatic schizonts of L. simondi undergo prolific development in phagocytic cells of the host which become grossly hypertrophied. These infected cells together with the parasites within are termed megaloschizonts. Young megaloschizonts contain numerous round cytomeres bounded by a five-layered envelope. With further development the cytomeres expand and become invaginated. Microtubular spindles occur in dividing parasite nuclei at this stage of development. Thickenings form on the single membrane bounding the newly formed cytomeric surfaces. Electron-dense paired organelles develop adjacent to the thickenings which become the polar rings of merozoites. Abundant granular endoplasmic reticulum and numerous mitochondria with vesicular cristae are seen in the cytoplasm of invaginated cytomeres. Nuclear division by multiple fission and further cytoplasmic fragmentation continue until uninucleate merozoites are formed. Male and female merozoites can be distinguished by the difference in the density of their cytoplasm and each contains a mitochondrion and paired organelles with the associated polar ring. The megaloschizont is surrounded by a capsule whose irregular inner surface is lined by the host plasma membrane. Outside this lies a thick filamentous zone from which numerous vesicles are pinched off and are seen within the hypertrophied host cytoplasm. A fibrous layer bounds the outer surface of the megaloschizonts.