[Light and electron microscope studies on cysts of Sarcocystis fusiformis in the muscles of calves infected experimentally with oocysts and sporocysts of Isospora hominis Railliet et Lucet, 1891. 2. Fine structure of metrocytes and merozoites (author's transl)].

In several experiments calves were infected with sporocysts of Isospora hominis thus producing "thick-walled" cysts of Sarcocystis fusiformis in muscles. Following the growth of the cyst the development of the cyst stages (metrocytes, merozoites) was studied by electron microscopy. Cyst formation began about 40 days p.i. from a parasitophorous vacuole containing exclusively ovoid metrocytes. On the 62nd day p.i. mainly metrocytes and a few banana-shaped merozoites were present within the cysts, whereas on the 98th day p.i. only merozoites, i.e. infectious stages, were observed. The metrocytes were surrounded directly by the amorphous ground substance of the cyst's interior, but the merozoites were arranged relatively loose within chamber-like hollows of the ground substance. The metrocytes are globular cells, about 6-7 mum by 4. 5 mum. The typical three-layered pellicle had a few invaginations and several micropores, which seem to ingest numerous small vesicles from the interior of the cyst. These cells posses a conoid, polar ring with 22 anchored supellicular microtubules, several rhoptries and micronemes, a glogi complex anterior to the large nucleus. The nucleus has a spherical nucleolus consisting of granular and fibrillar zones. Chromosomal structres were seen in two different stages: large dense plaques (condensed stage), and as small dense granules of 300-400 a diameter, arranged sperically within the karyoplasm ( extended stage). The nuclear pores show the typical eight-fold symmetry known from other protozoa and numerous metazoa. The rought endoplasmic reticulum is very prominent within these cells as well as the tubular mitochondria...