[Light optical and electron microscopy research on the cyst-forming coccidium Sarcocystis muris]

Part of the complicated life cycle of Sarcocystis muris, confined to the muscle cyst (sarcocyst), has been studied by light and electron microscopy. The early development of the sarcocyst proceeds strictly intracellularly, whereas the older and larger cysts tend to destroy the harbouring muscle cell, and since then their development seems to be intercellular rather than intracellular. Three different cell types are distinguished within the growing sarcocyst of S. muris differing from each other both structurally and functionally: metrocytes, intermediate cells and merozoites. These differ as well in the structure of their nuclei. The metrocyte nuclear chromatin is mainly in decondensed state with some minute granules taking the central part of the nucleus. The condensed chromatin of the intermediate cell is accumulated into some relatively large peripheral granules, whereas numerous RNP-granules appear in the karyolymph. The nuclear chromatin of merozoites is condensed to be seen as separate chromocenters scattered over the nucleus; the karyolymph is packed with RNP-granules. Metrocytes are seen to divide in young sarcocysts, although the mode of their division is still obscure. In sarcocysts of advanced age (2.5 months or more), only intermediate cells are seen to divide, their mode of division being endodyogeny.