[Light and electron microscope studies on cysts of Sarcocystis ovicanis Heydorn et al. (1975) within muscle fibres of sheep (author's transl)].

Four conventionally reared lambs, isolated at the age of 5 and 8 weeks, were orally infected with oocysts and sporocysts from dogs, which had been fed raw muscles from sheep containing small cysts of S. tenella. Three lambs, each infected with 100,000 sporocysts, were killed at days 41, 63 and 81 p.i. The other lamb was used for a non-infected control. The development of Sarcocystis-cysts in muscle cells of the infected lambs was studied by light and electron microscopy. The cyst was always situated within a muscle fiber which was never surrounded by fibrillar layers (=no secondary cyst wall). The cyst was limited by a unit membrane, which was thickened at numerous places of the interior by osmiophilic material. This complex is called primary cyst wall (= Primärhülle), reaching a thickness of up to 25 nm. In old cysts this primary wall was regulary folded, forming palisade-like protrusions of about 3.5 mu in length. In light microscopy the combined protrusions had the appearance of a radially striated "thick wall", because of their close proximity to each other. During formation of the palisade-like protrusions the thin areas of the primary wall were restricted to the base of the protrusions and to the small space between the protrusions. Here, the single unit membrane formed vesicle-like invaginations of about 40 nm in diameter into the interior of the cyst. Vesicles seen in the cysts were thought to derive from these invaginations. Within the palisade-like protrusions never fibrillar or tubular elements appeared. In comparing the fine structure of the cyst wall of the small cysts, studied here, with the macroscopically visible cysts we found significant differences. These differences in the morphology confirm the results of transmission experiments, by which it was shown that S. tenella as described in literature is part of at least two coccidian life cycles. So the term S. tenella was replaced by two new species: S. ovicanis (final host: dog) and S. ovifelis (final host: cat) Heydorn et al. (1975).