'Shaking pups': a disorder of central myelination in the spaniel dog. IV. Freeze-fracture electron microscopic studies of axons, oligodendrocytes and astrocytes in the spinal cord white matter.

Large axons identified in freeze-fracture replicas of the spinal cord white matter of 'shaking pups' were encircled by spiral processes at the paranodes that appeared to arise from oligodendrocytes. In the axolemma adjacent to some of these processes there were paranode-like intramembranous specializations and node-like accumulations of E-face particles; in other instances, no special contacts formed between axons and the spiral processes. The outer surfaces of the spiral processes, which often apposed astrocytes, frequently contained gap junctions. Although many of the abnormalities identified in this dysmyelinating mutant by freeze-fracture electron microscopy are probably secondary to a more fundamental defect of myelin formation, the prominence of the spiral processes suggests that the encircling of axons by oligodendrocytes may be an independent state of ensheathment and not a passive effect of myelin formation.