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 s taining. It is shown that this structure gradually acquires its defini tive appearance. When fully formed, it consists of a ribbon, about 52 mum long, that in resting condition is tightly rolled up around a cent ral 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 th e ribbon rapidly unrolls from the inside forming a hollow tube about 3 5 mum long and 150 nm in diameter. The tubular configuration and the r atio between the length and the diameter suggest that the tube has a g reat resistance that is independent of its chemical nature. A sort of ''hood' that, as previously demonstrated, contains DNA and proteins su rrounds 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 ul trastructural data show that the extrusive apparatus in these epixenos omes is a highly complex and efficient structure and strengthen the hy pothesis of a dispersive role played by the ejecting process.