Electron microscopic study of the division processes in cysts of the sarcosporidian, Sarcocystis, ovifelis

The metrocyte stage in the cysts of S. ovifelis has been found to divide not by endodyogeny, but by a different mode, not yet completely understood. The nucleus of the dividing metrocyte displays no nuclear envelope, the cytoplasm is full of large vacuoles. During the metrocyte division, some kind of nucleoplasm spreading occurs over the cytoplasm and between the large vacuoles, thus making any morphological limits between the cytoplasm and the nucleus invisible. The cytokinesis is accomplished due to the deep invaginations of the metrocyte pellicle. The metrocyte division first gives rise to a large multicellular (4--6) body lying in the peripheral zone of the cyst. The very impossibility of endodyogenetic division in the metrocyte stage may be due to its vaery peculiar morphofunctional organization, much different from what is characteristic of any asexual penetrative stage (= zoite) of Apicomplexa. The progeny of a metrocyte are interstitial cells which eventually, through the number of generations, evolve towards the metrozoite stage. Step by step, the interstitial cells aquire polarity due to the establishment of a conoid, rhoptries and micronnemes on the anterior pole, the pellicle becoming more rigid thus making, together with subpellicular microtubules, the cytoskeleton of the parasite. The study performed enables us to conclude that it is interstitial cells of advanced generations that divide by endodogeny. Merozoites being homologous to gamonts of other coccidia undergo no asexual division at all.