The distribution of axons according to diameter in the optic nerve and optic tract of the rat

Abstract

The distribution of axons according to diameter was examined in the optic nerve and optic tract of adult hooded rats. Observations were made on semithin sections, and measurements of axonal diameters were made on electron micrographs taken from various locations across thin sections through the optic nerve and tract. The distribution of axons by size differs markedly in the optic nerve and tract. Coarse (>2 μm) and fine (⩽2 μm) axons are distributed throughout all regions of the optic nerve. In the optic tract, in contrast, coarse axons are especially dense dorsally, at the deep border of the tract, while they are absent ventrally, subjacent to the pial surface. No regions of the optic nerve contain densities of coarse axons as high as the deep nor as low as the superficial extremes of the optic tract. Nevertheless, even at the deep (dorsal) border of the optic tract, the coarse axons make up only a small minority (roughly 15%) of the total number of axons in that region. The axons 2 μm or smaller may be divisible into two overlapping, fine and intermediate, diameter classes, that are partially segregated within the optic tract, but not in the optic nerve: the distributions of axon diameters smaller than 2 μm are skewed to distinctly smaller diameters at the dorsal and ventral extremes of the optic tract, while in between, at mid-positions along the deep-to-superficial axis of the optic tract, the axon size distributions contain many more axons greater than 1 μm in diameter. These different axon diameter groups may arise from the morphologically distinct retinal ganglion cell types, and may underlie the components of the trimodal compound axon potential seen in the rat's primary optic pathway. Their partial segregation within the tract anticipates the partial segregation of their terminal arborizations within the laminae of the dorsal lateral genkulate nucleus. The rearrangement of axons into a partial segregation by size within the optic tract may indicate a chronology of axonal arrival during early development, proximity to the pial surface being an index of recency of arrival. As axonal outgrowth and neurogenesis appear to be directly related within the retinal ganglion cell population in mammals, the relative birthdates of the retinal ganglion cell types giving rise to the axon diameter classes in the rat may be inferred from the present results.