Morphology of freeze-substituted myelinated axon in mouse peripheral nerves

In order to clarify the in situ shape of axons, the entire shape of freeze-substituted axons (FS axons) was compared with that of chemically-fixed axons (CF axons) using the serial semi-thin section method coupled with rapid freezing technique. Mouse saphenous nerves were rapidly frozen by being contacted against a pure copper block cooled to 4 K by liquid helium. At the internode, the FS axons showed a cylindrical outline while the CF axons showed a polygonal contour. At regions near the node of Ranvier, both FS and CF axons showed a cog-wheel contour, and at the node they became circular and highly attenuated. These findings indicated that the axons, at the internode, were artifactually distorted during chemical fixation and that the in situ shape of myelinated axons was characterized by the cylindrical internodal segment, the distorted juxtaparanodal segment and the attenuated nodal segment. The physiological meanings of the entire shape of axons were discussed with special reference to the molecular mechanism of slow axonal transport and membrane excitation.