Retinal ganglion cell axon diameter spectrum of the cat: mean axon diameter varies according to retinal position.

Abstract

Axon diameters of retinal ganglion cells were measured from electron micrographs of the nerve fiber layer of the cat. Three adult retinae were examined which had mean axonal diameters of 1.18 +/- 0.86 (n = 5553), 1.12 +/- 0.79 (n = 7265), and 1.47 +/- 1.11 microns (n = 10,867). Cumulative histograms from several locations adjacent to the optic disc were unimodal (modal peaks: 0.6-0.8 microns). This unimodal distribution, however, did not reflect the regional differences in axonal diameters found throughout the retina. In many locations, especially those related to axons of the temporal retina, axon diameter distributions were clearly bimodal or even trimodal (modal peaks: 0.6-0.8, 1.4-2.1, and 3.3 microns). Measurements from one retina indicated that the mean diameters of axons arising from the area centralis and visual streak (0.94 +/- 0.63 and 0.98 +/- 0.68, respectively) were not significantly different from each other; however, when compared to other areas around the optic disc, the percentage of fibers with diameters between 1.5-2.0 microns was highest in the sample adjacent to the area centralis. Axons temporal to the optic disc were found to be on average larger than those nasal to the optic disc; similarly superior axons were larger than inferior axons. Axonal distributions at the retinal periphery were found to be significantly different from those at the optic disc (P < or = 0.05) and contained a higher percentage of medium-sized axons and fewer small axons. In each of the three retinae the proportions small, medium, and large axons were respectively gamma: 46; 47; 48, beta: 50; 49; 48, and alpha: 4; 4; 4; regional differences in the proportions of each axonal class are compared to previously published ganglion cell density maps. Differences between axonal bundles within each sample location were not significantly different; however, in one retina axons in the scleral half of the fiber layer were significantly larger (P < or = 0.01) than axons in the vitreal half of the nerve fiber layer adjacent to the optic disc. When compared to the axonal diameter distributions found within the optic nerve (Cottee et al., 1991) and optic tract (Reese et al., 1991), our data indicates that the diameter of retinal axons may increase by up to 30% along the length of the visual pathway.