The Cyst Wall Formation in Tillina magna (Ciliophora, Colpodidae)

Abstract

Summary Structure and formation of the resting cyst wall of Tillina magna were studied using electron microscopy. The cyst wall of this ciliate is formed by exocytosis. In mature cyst, it consists of three layers. The filamentous framework of every layer ist probably formed by mucocysts. Beside that, all the layers have their own specific precursors. On the basis of continuing formation of the whole thick filamentous coat, the cell successively excretes small (30–50 nm) electron dense granules, then large (250–400 nm in diameter), also very dense, spherical bodies, and, at last, material in a form of irregular clots. All these structural elements are inserted into the cyst wall and form three dense layers partitioned off by more loose purely filamentous material. The differences in density may cause the wall splitting along the borders of dense layers. The typical parts of the cyst wall may be designated as ectocyst, mesocyst and endocyst. Furthermore, the cell also excretes granules not forming a continuous layer and included into the mesocyst as separate bodies. At the beginning of encystment, many food and autophagous vacuoles fuse and form a large vacuole, its contents being extruded from the cell. Cilia seem to be retained in the cyst without any significant alteration in structure.