[Light and electron microscopic studies on cysts of Sarcocystis fusiformis in the muscles of calves infected experimentally with oocytes and sporocysts of the large form of Isospora bigemina from dogs. 2. The fine structure of cyst stages].

In several experiments calves were infected with sporocytes of the large form of Isospora bigemina from dogs thus producing "thin-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 34 days p.i. from a parasitophorus vacuole containing exclusively ovoid metrocytes. One the 62nd day p.i. mainly metrocytes and a few banana-shaped stages were present in the cysts. These banana-shaped stages were called merozoites, because we consider the process of cyst formation as an extraintestinal schizogony. From the 76th day p.i. mainly merozoites, i.e. infectious stages, were observed. The metrocytes were surrounded directly by the amorphous ground substance of the cyst's interior, whereas the merozoites were closely packed within chamber-like hollows of the ground substance. The metrocytes are globular cells, about 7 X 5 mum. These cells possess as the metrocytes of other species a typical three-layered pellicle with deep micropores, a conoid, polar ring with 22 anchored subpellicular microtubules, very few rhoptries and micronemes, a golgi complex anterior to the large nucleus. The nucleus has a spherical nucleolus consisting of granular and fibrillar zones. Chromosomal structures were seen in two different stages: large dense plaques (condensed stage), and as small dense granules of 300-400 A diameter, arranged spherically within the karyoplasm (extended stage). There is no significant difference in fine structure between the metrocytes of I. hominis-induced cysts and those from cysts after infections with the large form of I. bigemina from dog.