[Electron microscopic investigation on the evolution stages of the trophozoite of Didymophyes gigantea (Sporozoa, Gregarinida). I. The fine structure of the proto- and epimerite and the relationship between host and parasite (author's transl)].

The fine structure of the epimerite and the protomerite of D. gigantea was investigated following the different stages of the evolution of the trophozoite. The first stage is mononucleate and very poor of cytoplasmic structures (Fig. 1). All following stages are polynucleate (Fig. 3). The septum between protomerite and deutomerite appears at the third stage (Fig. 22). The body of the gregarine is in all stages surrounded by a thick cell-wall constituted of three membranes. The limit between epimerite and protomerite is indicated by the existence of an osmiophilic basal ring just under the parasite's cell-wall. This basal ring has a transverse substructure with a periodical distance of 450 A (Fig. 12). The two inner membranes join together just beneath the basal ring, whereas the outer membrane continues and forms the single cell membrane of the epimerite (Fig.9). The epimerite displays a large number of microvillosities and evaginations of the membrane which increase considerably its surface (Figs. 11-14). The epimerite essentially contains ribosomes and mitochondria (Fig.6). The protomerite contains in addition to the usual cell organites cytoplasmic agglomerations consisting of endoplasmic reticulum, ribonucleoprotein and peripheric mitochondria. During evolution from stage IV to V, these agglomerations produce osmiophilic granula of different form and size, most of which are surrounded by an elementary membrane (Fig. 18). Beside these granula one can observe sometimes small spherical or lamellar structures (Fig. 21). Large bundles of microtubules run from the protomerite into the epimerite where they generally branch off and penetrate into the mitochondrial layer (Figs. 6, 19, 20). Between the parasite and the host-cell exists a space, the periparasitic space, without any cytoplasmic inclusions. The cell membrane of the host-cell has in the periparasitic space an outer cell-coat with a ripped pattern (Fig. 13). The microvillosities of the cell membrane disappear at this place. The host-cell cytoplasm displays a large number of microtubules and a fibrillar network (Figs. 13, 14). The importance of these structures for the relationship between parasite and host-cell is discussed.