Licht- und elektronenmikroskopische Untersuchungen an Entwicklungsstadien von Sarcocystis tenella aus der Darmwand der Hauskatze

Summary

In several experiments young cats were infected with Sarcocystis tenella cysts from the esophagus of sheep and the stages in the cat intestine were examined by means of light and electron microscopy. Eleven to twelve days after infection untreated cats excreted a very small number of fully sporulated sporocysts, which measured 12.4×8.7 μm (14.0–10.5 μm×9.7–8.0 μm). Oocysts were not observed here. Sporocysts could be found even 14 days after the beginning of the excretion. The sporocysts, which had no stieda-body, contained 4 sporozoites and a residual body, thus corresponding to the Isospora-type. In a further experiment a cat was treated for 4 weeks with a corticosteroid (2.5 mg daily, Prednison) before infection. This cat also was fed approximately 100 large cysts of S. tenella, which had been carefully extracted from the esophagus. On the 9th day p.i. the cat was killed and the intestine was examined. The latter showed macroscopically no hemorrhage, but contained a considerable number of very thin-walled oocysts without a micropyle. A remarkable accumulation of oocysts was seen in the area of the posterior small intestine. Almost 90% of the oocysts already contained two sporocysts, in which large, dark or light granules were found like those in the few unsporulated oocysts. The sporulated oocysts measured about 18.0–14.8 μm×14.0–10.5 μm. The sporocysts in this experiment were, with 10.9×6.8 μm (12.4–9.8 μm×8.0–6.2 μm), slightly smaller than those of the earlier experiments. Light microscopical investigations of cross sections of the intestine showed that the parasites were always situated immediately under the epithelial cells in the villi of the intestine. They were also always seen in large parasitophorous vacuoles. The oocysts were found relatively often to form long rows, thus indicating that the parasitized host cells were possibly sunken epithelial cells. Electron microscopical studies showed that the unsporulated oocysts were surrounded by a single, very electron-pale wall, which had at this time a diameter of about 0.25 μm and which shrank during sporulation to 0.1 μm. The cytoplasm of the oocyst was limited by a typical unit membrane and contained large reserve granules (polysaccharides, lipids) with a diameter of about 1.5 μm. In addition small spherical elements (30–40 nm) were found, which formed a cristalline pattern. 0.4 μm sized osmiophilic bodies are probably responsible for the formation of the later sporocyst walls. The large nucleus was often situated at the periphery of the spherical unsporulated oocyst, which also possessed numerous tubular mitochondria. Occasionally micropores were still present as relics in the cytoplasmic membrane. The sporocysts were limited by a 50–60 nm thick osmiophilic layer and contained the same elements as the unsporulated oocyst. Most of the ovoid sporocysts had two u-shaped nuclei at the poles. Sporulated as well as unsporulated oocysts were always situated in a large electron-pale parasitophorous vacuole. At this time the host cell consisted only of two remaining membranes: the outer cytoplasmic membrane and the limiting membrane of the parasitophorous vacuole. Therefore it was not possible to decide which cell-type had been parasitized. The oocyst wall proved very fragile in the sporulated stage, so that often single free sporocysts were observed. The free sporocysts had, in addition to their sporocyst wall, a thin granular outer layer, which may be considered a relic of the oocyst wall. From the ultrastructure of the oocyst wall and the advanced state of sporulation (in the tissue) on the 9th day p.i., it becomes clear why, in the transmission experiments, only fully sporulated sporocysts were found in the feces on the 11th–12th day. Finally our results were compared with the other Isospora species of cat. Apparently the large form of Isospora bigemina seems responsible for the relatively large cysts in the muscles of sheep, described formerly as S. tenella.

Zusammenfassung

In mehreren Versuchen wurden junge Katzen mit Cysten von Sarcocystis tenella aus der Oesophagus-Muskulatur von Schafen infiziert und die Stadien im Katzendarm licht- und clektronenoptisch untersucht. Bei den Übertragungsversuchen ergab sich, daß vom 11.–12. Tag p.i. Sporocysten ausgeschieden wurden, die in ihrem Innern 4 Sporozoiten und einen großen Restkörper enthielten. Ein Stieda-Körper fehlte diesen etwa 12,4 × 8,7 μm großen Sporocysten, die für lange Zeit in sehr geringer Anzahl abgesetzt wurden. Bei einer mit einem Corticosteroid (Prednison) vorbehandelten Katze konnte am 9. Tag p.i. im Darminnern eine starke Anhäufung von Parasiten beobachtet werden. Hier fanden sich zahlreiche dünnwandige Oocysten, die meist schon zwei Sporocysten im Innern aufwiesen. Die Oocysten lagen oft in langen Reihen unmittelbar unterhalb der Epithelzellen der Darmvilli innerhalb von großen, lichten parasitophoren Vakuolen. Der Feinbau der unsporulierten Oocysten und der Sporocysten wurde beschrieben und mit den Verhältnissen bei Eimeria-Arten verglichen. Aus der Ultrastruktur der Oocystenhülle und dem Sporulationsstand am 9. Tag p.i. geht hervor, warum bei anderen und unseren Übertragungsversuchen stets völlig sporulierte Sporocysten am 11.–12. p.i. im Kot angetroffen wurden. Im Vergleich mit den anderen Isospora-Arten der Katze zeigte sich, daß offenbar die große Form von I. bigemina für die ziemlich großen Cysten in der Muskulatur von Schafen verantwortlich ist.