Filamentous and multilamellated cytoplasmic inclusions in progressive multifocal leukoencephalopathy

Inclusions identical to those described in multiple sclerosis (MS) brain by Prineas (1975) have been seen by ultrastructural study of cerebral tissue in two elsewhere reported cases of Progressive Multifocal Leukoencephalopathy (PML). The meaning of these formations, which have been suggested to be related to a special process of demyelination in MS, is discussed in the light of the hypotheses concerning the demyelination mechanism in PML. Whatever might be their significance, these cytoplasmic non viral inclusions are not specific for MS.