An electron microscope study of quantitative relationships between axon and Schwann cell sheath in myelinated fibres of peripheral nerves.

The quantitative relationships between the cross-sectional area of the Schwann cell sheath (myelin included) and that of its related axon were studied by electron microscopy in the nerve fibres of the spinal roots of lizard (Lacerta muralis). In both ventral and dorsal roots the cross-sectional area of the Schwann cell sheath (myelin included) was found to be directly proportional to that of its related axon (correlation coefficients between 0.88 and 0.92). The ratio between the cross-sectional area of the Schwann cell sheath (myelin included) and that of its related axon tends to diminish as the cross-sectional area of the latter increases. Thus, under normal conditions, in myelinated fibres of the spinal roots of the lizard a quantitative balance exists between the nerve tissue and its associated glial tissue. This result agrees with those previously obtained in the spinal ganglia of the lizard, gecko, cat and rabbit. Some of the mechanisms probably involved in the control of the quantitative balance between nerve tissue and its associated glial tissue in peripheral nerves are presented and discussed.