Current conception of the Sarcosporidia (Sarcocystis, Eimeriidae, Sporozoa, Apicomplexa): their morphofunctional organization, life cycle and practical importance

Advances in sarcosporidian research within the latest decade are critically reviewed and analysed in respect to some recent cytological findings of the author and her colleagues on the subject. Three rather than two morpho-functional cell types (metrocytes and merozoites) are distinguished within the sarcocyst, the third one being the intermediate cell. Division by endodyogeny in the cyst of Sarcocystis is very likely confined to the latter cell type. The pattern of nuclear chromatin and the constancy in DNA value per nucleus in the cystic merozoites, revealed by flow cytometry, is rather indicative of their incapability of dividing within the cyst, i.e. in the intermediate host. This enabled us to consider these merozoites as homologs of coccidian gamonts. The obvious differences between cysts and cystic stages in Sarcocystis and Toxoplasma may account in part for the rare, if any, reported cases of congenital sarcocystosis in the intermediate host.