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Tsitologiia. 2001;43(11):1005-12.

[Oocyst structure and problem of coccidian taxonomy].

[Article in Russian]

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  • 1Institute of Cytology RAS, St. Petersburg.


A comparative ultrastructural study was made of both thin- and thick-walled oocysts of Cryptosporidium parvum. According to the authors' findings, all the oocysts in C. parvum should be considered as thin-walled, since their walls have been composed of a single membrane or of two, closely apposed membranes without any additional substance in between. Despite the presence of two types of wall-forming bodies (WFB) in the maturing macrogamete or zygote, there is no evidence of their involvement in oocyst wall formation. In this concern, the function and destiny of WFB in C. parvum oocysts still remain obscure. Similar structure of the oocysts wall was reported elsewhere for thin-walled oocysts of fish coccidia of the genera Goussia and Eimeria. In C. parvum, the "thick-walled" oocysts differ from oocysts with thin walls in the availability in the former of a single sporocyst. The sporocyst wall consists of two unequal layers: a thin outer layer and a thicker inner one, in which a characteristic suture line is occasionally seen. By this feature the thick-walled oocysts of C. parvum bear similarities with oocysts of the cyst-forming coccidia (Cystoisospora, Toxoplasma, Sarcocystis) and of the genus Goussia: in all these the valves making up the sporocyst wall are joint just along the suture line. The literary and the authors' own data make it possible to suppose that the suture detected in C. parvum oocysts is located in the sporocyst wall, joining its valves, rather than in the oocyst wall proper, known to be composed of one or two, closely apposed unit membranes. Again, the availability of a suture (or sutures) in the sporocyst hardly provides enough reason to relate C. parvum with either cyst-forming, or fish coccidia, since this structure itself may be of a convergency character, rather than of systematic value. This may be substantiated, at least in part, by the authors' previous findings (Beyer, Sidorenko, 1984) of a similar structure, originally referred to as a "slit channel", in the intraerythrocytic capsule around gamont stage of haemogregarines--the adeleid coccidia of the genus Karyolysus. The suture-like structure could have originated in the evolution independently in different groups of parasitic protozoa to serve eventually as a suitable mechanism for immediate separation of elements involved in protective formation harbouring different developmental stages, including, for example, sporozoites in the eimeriid coccidia, or gamonts in the adeleid coccidia.

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