The in vitro production of gametocytes and oocysts of the apicomplexan parasite genus Eimeria is still a challenge in coccidiosis research. Until today, an in vitro development of gametocytes or oocysts had only been shown in some Eimeria species. For several mammalian Eimeria species, partial developments could be achieved in different cell types, but a development up to gametocytes or oocysts is still lacking. This study compares several permanent cell lines with primary fetal cells of the black rat (Rattus norvegicus) concerning the qualitative in vitro development of the rat parasite Eimeria nieschulzi. With the help of transgenic parasites, the developmental progress was documented. The selected Eimeria nieschulzi strain constitutively expresses the yellow fluorescent protein and a macrogamont specific upregulated red tandem dimer tomato. In the majority of all investigated host cells the development stopped at the second merozoite stage. In a mixed culture of cells derived from inner fetal organs the development of schizont generations I-IV, macrogamonts, and oocysts were observed in crypt-like organoid structures. Microgamonts and microgametes could not be observed and oocysts did not sporulate under air supply. By immunohistology, we could confirm that wild-type E. nieschulzi stages can be found in the crypts of the small intestine. The results of this study may be helpful for characterization of native host cells and for development of an in vitro cultivation system for Eimeria species. 1. Introduction The parasites of the genus Eimeria can be found in almost all vertebrates and some species are of high economical relevance as they can cause coccidiosis in livestock. Eimeria parasites are known to have a stringent stage progression in their life cycles in the host animals. They develop mainly in the intestinal tract, but some species are also found in the liver (rabbit) or kidney (goose). To study all stages of the parasites, it is helpful to have the complete development in vitro. In primary cells, only a few Eimeria species are able to form gametocytes or complete their life cycles in vitro [1]. Often, this is only possible if merozoites are used for infection [2–4]. For rodent Eimeria species, which are frequently used as model organisms, a gametocyte or oocyst development was not achieved thus far. In previous studies with the rat parasite Eimeria nieschulzi, the stage development in vitro did not exceed the second merozoite stage [5–7]. Whereas, up to four schizont generations are found in the host before an in vivo gamont development
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