Myelinated axon undergoes complete demyelination in the panencephalopathic--but it is merely subjected to the Wallerian degeneration in the polioencephalopathic type of transmissible spongiform encephalopathies.

We report here on axon and myelin changes in panencephalopathic type of Creutzfeldt-Jakob disease as opposed to polioencephalopathic models of scrapie. In CJD, myelinated axons presented various pathological changes. Initially the myelin sheath was separated by cytoplasmic tongues into several concentric bands. Cellular processes penetrated between layers of myelin and lifted away the outermost lamellae. Then a complicated labyrinth of concentric cellular processes, clearly belonging to either astrocytes or macrophages invested myelinated axons. In terminal stages axons completely denuded of myelin were seen in the centre of a concentric network of cellular processes or spirals of myelin were seen surrounded by such processes. The myelin fragments penetrated into astrocytes or macrophages where they underwent final digestion. Macrophages containing fragments of axons, paracrystalline lamellar bodies and myelin debris were abundant in this model. In two models of scrapie myelin dilatation, not unlike that seen in the panencephalopathic type of CJD was observed. Several altered axons were also seen, but these presented only typical features of Wallerian degeneration. Even the most damaged fibres consisted of "empty" vacuoles (the axon itself was lost) surrounded by a narrow rim of oligodendroglial cytoplasm. Thus, myelinated fibre degeneration in this polioencephalopathic model merely represents the sequel to preceding neuronal degeneration. In conclusion, it seems that myelinated fibres are eventually denuded of myelin in the panencephalopathic type of CJD but undergo Wallerian degeneration in polioencephalopathic types of scrapie. Because myelin dilatation is observed in both pan- and polioencephalopathic type of TSE and because their formation is probably mediated by cytokines released from astrocytes and microglia, we hypothesised that this mechanism operates early in the fibre destruction and must be supplemented later in the course of the disease by other, currently unknown mechanisms.