The number and diameter distribution of axons in the monkey optic nerve

Using an automated image analysis system, cross-sections from optic nerves of 17 normal cynomolgus monkeys were examined. The number of nerve fibers, their density, and the distribution of their diameters for whole nerves and for various regions of the nerve cross-section were estimated. The mean total number of fibers in the optic nerve was 1.2 million. The mean diameter of axons was 0.8 μm. The method of tissue fixation substantially affected the measurements. Histograms of fiber diameter suggested a trimodal distributon of fiber size with peaks at 0.5, 0.8, and 1.5 μm. The relative proportions of these fiber peaks differed significantly in different regions of the nerve. The highest proportion of large fibers was in the superior nerve periphery. The highest concentration of smallest fibers and the highest density of all fibers were located centrally in the infero-temporal quadrant. The observation that higher fiber density and smaller mean fiber diameter are skewed toward the inferior pole appears to coincide with the inferior position of the fovea with respect to the optic nerve head. This finding has importance for interpretation of pathologic changes in the optic disc.