"Epixenosomes": Peculiar epibionts of the ciliate Euplotidium itoi: The formation of the extrusive apparatus and the ejecting mechanism.

The ultrastructure of the extrusive apparatus of epixenosomes has been analyzed by means of SEM observations, thin sectioning and negative staining. It is shown that this structure gradually acquires its definitive appearance. When fully formed, it consists of a ribbon, about 52 μm long, that in resting condition is tightly rolled up around a central core 150 nm in diameter. A cytochemical analysis shows that it is immersed in a proteic matrix different from the remaining cytoplasm. A network of 20 nm fibrils surrounds the ribbon. These fibrils may have an important role during the ejecting process. During the ejection the ribbon rapidly unrolls from the inside forming a hollow tube about 35 μm long and 150 nm in diameter. The tubular configuration and the ratio between the length and the diameter suggest that the tube has a great resistance that is independent of its chemical nature. A sort of "hood" that, as previously demonstrated, contains DNA and proteins surrounds the distal part of the ejected tube. A bundle of 20 nm fibrils emerging from its lumen forms the very tip of the structure. These ultrastructural data show that the extrusive apparatus in these epixenosomes is a highly complex and efficient structure and strengthen the hypothesis of a dispersive role played by the ejecting process.