Intermittent myelination of small-diameter sciatic axons in Xenopus laevis

Nerve fibres from the sciatic nerve of apparently normal Xenopus laevis were examined in teased preparations of living material and in series of cross-sections. Small-diameter fibres were found that were myelinated in a spatially intermittent fashion. In these fibres, the myelinated segments were separated by long non-myelinated regions that had a 1 : 1 relationship to the investing Schwann cell. It is estimated that at least 10% of the myelinated fibres with an external diameter of less than 5 micrograms were intermittently myelinated.