[Current views on the life cycle of cyst-forming Coccidia (Eucoccidia, Sporozoa, Apicomplexa)].
The life cycles of cyst-forming coccidia of the genera Sarcocystis and Toxoplasma have been first analysed in terms of generally recognized biological phenomena, such as proliferation, differentiation, dedifferentiation, programmed cell death. The differences, existing between the respective obligatory heteroxenous (Sarcocystis) and facultatively heteroxenous (Toxoplasma) life cycles, have been found to involve the obvious differences in the degree of a zoite's differentiation, occurring in the course of asexual development in the intermediate host. In Sarcocystis spp., the degree of differentiation in merozoites, throughout their development, is much higher than in those of T. gondii. This level of merozoite differentiation in Sarcocystis is thought to determine the irreversible, one-directional way of both pre-cystic and cystic development of the pathogen, starting from the sporozoite stage and terminating in gamont formation within the tissue cyst. Unlike, in T. gondii, the level of merozoite differentiation is not so strong as in Sarcocystis spp., which may account for the reversible merozoite development in the former, which is clearly demonstrated by a ready conversion of pre-cystic tachyzoites into cystic bradyzoites, and the other way round. In the course of endogenous development, the pathogens adversely affect their environment (the infected cells and tissues), which, in its turn, may exert its influence on the particular parasites. Thus, both the parasite and the host represent a unique feed-bach regulatory system.
Radchenko
A I
AI
Institute of Cytology, Russian Academy of Sciences, St. Petersburg.
D000818:Animals; D003044:Coccidia / Q000254:growth & development*; D008018:Life Cycle Stages