Distribution of axons according to diameter in the monkey's optic tract

Abstract

The distribution of axonal diameters in the optic tract of Old World monkeys was examined by light and electron microscopy. Axon diameters were measured in samples of 100 axons taken from several locations in a cross section of the tract about 5 mm behind the optic chiasm. Fine‐caliber axons (2.5 μm) were found in the most ventral parts of the tract, near the pial surface. This pattern of segregation of axons of differing diameters in the optic tract is a rearrangement of the distribution of axon diameters seen in the nerve rather than a continuation of the same pattern. Examination of axon diameters in the optic nerve has shown that there is a preponderance of fine axons centrally, while coarser axons are found in the periphery, near the pial surface; however, histograms from central parts of the nerve contain a greater proportion of coarse axons than the dorsal parts of the optic tract, while histograms from the periphery of the optic nerve contain a conspicuously greater proportion of fine axons than do histograms from the most ventral parts of the tract. This relatively greater segregation of axons according to diameter in the optic tract demonstrates that the distribution of axons in the tract cannot be formed by the simple combination of two hemiretinal maps contained in each optic nerve, as suggested in classic descriptions. The optic axons of different diameters are likely to arise from morphologically distinct retinal ganglion cell types. Previous studies in the cat have demonstrated that axons of differing diameter classes, which arise from the distinct retinal ganglion cell types, are distributed in a uniform fashion in the optic nerve but are partially segregated according to their diameter in the tract. This rearrangement of axons between the nerve and tract in carnivores has been related to the temporal gradient of neurogenesis of the different retinal ganglion cell classes, axonal order in the optic tract being an index of axonal arrival during development. We suggest that the same occurs in the development of the monkey's retinofugal system.