Aberrant axon-Schwann cell junctions in dystrophic mouse nerves.

'Amyelinated' axons in the spinal roots of dystrophic mouse nerves lack typical nodal and paranodal membrane specializations. However, at the periphery of the amyelinated bundles some of the naked axons form aberrant junctions with Schwann cells belonging to neighbouring myelinated axons. These junctions are characterized by a narrow intercellular cleft containing regularly-spaced densities that closely resemble the 'transverse bands' found at paranodal axoglial junctions with respect to both configuration and spacing. In addition, the Schwann cells sometimes extend fingerlike projections towards amyelinated axons in regions where the axolemma has a dense cytoplasmic undercoating. Such regions resemble nodes of Ranvier, where Schwann cell processes interlace over the axolemma. Freeze-fracture replicas show no typical nodal or paranodal membrane specializations in the amyelinated fibres where they are apposed to each other. However, isolated paracrystalline patches of membrane occur corresponding to the aberrant junctions between amyelinated axons and Schwann cells at the periphery of the bundles. The observations show that structural differentiation of the axolemma occurs only where axons are in intimate contact with myelinating cells and does not develop independently in the amyelinated regions. Sodium channels, which are normally concentrated in the specialized nodal membrane, are, therefore, probably distributed uniformly along the amyelinated axon segments that show no sign of such regional differentiation. In addition, it is shown that Schwann cells are capable of forming specialized junctions with more than one axon at the same time.