Nerve fibres with myelinated and unmyelinated portions in dorsal spinal roots

Series of 312-605 consecutive sections were prepared from apparently normal dorsal spinal roots of lizard (Lacerta muralis). Two axons which showed a segment enveloped by a compact myelin sheath and a segment devoid of myelin were followed in these serial sections. These provided the opportunity of analysing the structural features that an individual axon presents when it is myelinated and when it is devoid of myelin. Some structural features (e.g., axon calibre and microtubule density) were significantly different in the myelinated segment and in the unmyelinated segment. The factors which possibly influence these features are briefly discussed.