Automated measurement of peripheral nerve fibres in transverse section

Transverse sections of rat tibial nerve were scanned with an automated flying-spot microscope and the pictures examined in a one bit (black-and-white) form. Computer programs have been developed, and are described briefly, to isolate individual nerve fibres in the field of view, and to measure axon area and myelin area, from which it is trivial to calculate average axon diameter and average myelin thickness if the axon is assumed to be circular in cross-section. At the magnification used, it was possible to measure 40-50 fibres per field and the time required to do this was 5-7 minutes. For each specimen, 7-8 fields were measured, providing data on 300-400 fibres. Measurements, made on nerves from rats between 3 and 38 weeks of age, show the rate at which nerve fibres grow and also show that the ratio between axon diameter and fibre diameter remains constant, within experimental error, over this period and is close to the predicted value.