Cytochemical studies on nuclear DNA of four eucoccidian parasites, Isospora (Toxoplasma) gondii, Eimeria tenella, Sarcocystis cruzi and Plasmodium berghei.

Feulgen-pararosaniline (SO2) staining was performed on stages in the life-cycle of Isospora (Toxoplasma) gondii, Eimeria tenella, Sarcocystis cruzi and Plasmodium berghei. The fluorescence emission of the stained DNA in nuclei of these stages was examined and compared with absorption microscopy measurements at 560 nm (green light) of the same specimens. Accurate identification of single cells, and especially discrimination between young schizonts and young gamonts was difficult after Feulgen staining. To overcome this problem preparations were first stained with Giemsa and the cells of interest precisely located with the aid of an England finder. The same preparations were then hydrolysed and stained with Feulgen-pararosaniline (SO2), after which the previously identified cells were investigated. The DNA distribution after Feulgen staining corresponded with the shape of nuclei after Giemsa staining. DNA was present in all investigated stages of the four parasites, including macrogamonts of I. (T.) gondii and E. tenella and peripheral blood gamonts of P. berghei. Macrogamonts could be recognized, even at a stage at which they can hardly be differentiated from young schizonts in Giemsa-stained preparations, by their typical distribution pattern of Feulgen fluorescence. Feulgen fluorescence was more granular and confined to the peripheral region of the nucleus, leaving a distinct nucleolus unstained. The horseshoe-shaped nuclei typical of macrogamonts could also be observed in some second generation merozoites of E. tenella, indicating that these merozoites are already sexually differentiated. The relationship between the present cytochemical observations and the ultrastructure of the four investigated parasites is discussed.