Elektronenmikroskopische Untersuchungen an Cystenstadien von Sarcocystis tenella aus der Oesophagus-Muskulatur des Schafes

Summary

The fine structure of the cyst stages of Sarcocystis tenella was studied by electron microscopy. The ovoid cysts were collected by macroscopic examination of the muscular region of the sheep oesophagus. The size of the cysts varies from 0.4–15 mm. The wall is composed of two distinct layers: a primary wall and a secondary wall.

The secondary wall consists of two layers. Outermost is a zone of lamellar cytoplasm and internal to this a zone of muscular tissue; in young cysts this zone has the typical aspect of muscular cells, with many bundles of myofibrils. Many nuclei and mitochondria are found in this zone. In larger cysts, however, the myofibrils are almost completely disintegrated; some remnants can still be seen. The so-called primary wall delimits the cyst proper. This osmiophilic layer, about 250 Å in diameter, forms numerous villus-like folds and vesicle-like invaginations into the interior of the cyst. At these places, the layer is reduced to a single membrane. Just beneath this limiting layer, many single myofibrils are observed in the granular ground substance of the cyst's interior, indicating that the infectious stage of S. tenella had invaded a typical muscular cell of the host. Within this ground substance, two different stages of S. tenella are seen in small cysts.

In the peripheral region, metrocytes are situated, whereas the interior is filled with banana-shaped merozoites (=zoites). The metrocytes are globular cells, about 15–20 μ long. The typical merozoite pellicle forms invaginations and several micropores. For the first time, it is demonstrated in serial sections that these cells possess a typical conoid, polar ring with anchored microtubules, and a Golgi complex anterior to the nucleus. Rhoptries (paired organelles) and micronemes are not observed. The nucleus has a nucleolus and a globular accumulation of electron dense granules (not seen in merozoites). By endodyogeny, two daughter cells are formed within these metrocytes. Clusters of metrocytes develop in this way, lying close together, with wavy cell borders. After several endodyogenies, the developing cells become progressively more similar to the later banana-shaped zoites, found in older cysts, in which almost no reproduction takes place. The merozoites measure about 12–15 μ in length, with a diameter of about 3–4 μ. Under the typical pellicle, we found 22 subpellicular microtubules. Unexpectedly, we observed 11 rib-like elements, evidently lying on the surface of the parasite. These “ribs” consist of rows of granules. The apical pole is characterized by the conoid, regular rows of many micronemes, and about 11 rhoptries. In some cases 3 rhoptries are observed to fuse at the apical pole of the conoid, with a vesicle-like appearance in cross section. The conoidal rings, the conoid, and the ductules are apparently connected by filamentous elements, because in all positions of the conoid all of these elements are in the same relation to each other. In an invagination at the apical pole of the nucleus, numerous filamentous elements with a characteristic striation are observed. This structure may be related to the origin of the centrioles, which were found during nuclear divisions at this location. The merozoites possess a single mitochondrion, which is branched giving a ring-like appearance.

Zusammenfassung

Die Feinstruktur der Cystenstadien von Sarcocystis tenella wurde untersucht. Die Cystenhülle besteht aus einer Primär- und einer Sekundärhülle. Die Primärhülle ist eine etwa 250 Å dicke, membranartige Schicht, die die eigentliche Cyste begrenzt. Die Sekundärhülle setzt sich aus einer äußeren lamellären Cytoplasmaschicht und einer inneren, Myofibrillen enthaltenden Cytoplasmazone zusammen. Primäre und sekundäre Schicht zeigen starke gemeinsame Verzahnungen. Das Cysteninnere besteht aus zwei Zelltypen (Metrocyten und Merozoiten), die in einer stark granulierten cytoplasmatischen Grundsubstanz liegen, in der ebenfalls Myofibrillen anzutreffen sind. Die Metrocyten liegen vorwiegend randständig und sind von ovoider Gestalt. Sie besitzen eine Merozoitenpellikula mit tiefen Einfaltungen und ein typisches Conoid. Die Merozoiten (Zoiten) sind meistens sichelförmig gebogen und besitzen alle typischen Merozoitenmerkmale. Ihr Conoid zeigt auf Längsschnitten etwa 10 spiralig gewundene Mikrotubuli. Auf Querschnitten lassen sich 22 subpellikuläre Mikrotubuli erkennen. Bis zu 11 kugelförmige Rhoptrienanschnitte konnten auf einem Längsschnitt gezählt werden. In einer Kernfurche am apikalen Kernpol befindet sich eine Cytoplasmazone mit einer periodischen Querstreifung. Eine solche Struktur wurde bei Coccidien bisher noch nicht beobachtet.