An electronmicroscopic analysis of the optic nerve in the golden hamster

The electronmicroscopic examination of sections taken from the hamster's optic nerve 5 mm behind the globe indicated that the nerve contains 110,165 +/- 4,177 (p less than 0.05) fibres of which 96.4% are myelinated. The fibre diameter distribution is unimodal with a peak at 1.2 micrometer and axon diameters ranging from 0.20 micrometer to 3.93 micrometer. Fibres of all sizes are distributed uniformly throughout the cross section of the nerve. The thickness of the myelin sheath surrounding a given axon is highly (0.80) correlated with axonal diameter and the degree of myelination for a fibre of a given size is nearly constant throughout the nerve's cross section. In nerve sections taken just posterior to the globe most (64%) of the fibres counted are unmyelinated and the percentage of unmyelinated axons is highest near the peripheral boundary of the nerve. The process of myelination is essentially complete in sections taken 3.5 mm behind the eye. These differences in the myelination of the proximal and distal nerve most probably account for the discrepancy between the results reported here and those provided by a previous study (Tiao and Blakemore, '76) concerned with the structure of the optic nerve in this species.