Tissues collected from Japanese eel elver (Anguilla japonica) experimentally infected with Pleistophora anguillarum were cultivated in Leiboritz's L-15 medium. A cell line designated as EP-1 (eel cells persistently infected with P. anguillarum) was established, and was found to be persistently infected with P. anguillarum. With this culture system, we first described the merogonial reproduction of P. anguillarum. Using histochemical and indirect immunofluorescent technique, it was revealed that there were 3-4 merozoites of different sizes residing in the cytoplasm of EP-1 cells. Electron micrographs of EP-1 cells showed that the merogonial stages had isolated nuclei (monokaryotic). The merozoite plasmalemma was separated from host cytoplasm by a membrane of host origin. The results of the infection experiment revealed that the present in vitro cultivation system could provide infective merozoites which were capable of proceeding to further sporogonial reproduction in the muscle of their natural host, eels.
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