RETINAL GANGLION-CELL AXON DIAMETER SPECTRUM OF THE CAT - MEAN AXON DIAMETER VARIES ACCORDING TO RETINAL POSITION

Axon diameters of retinal ganglion cells were measured from electron m icrographs of the nerve fiber layer of the cat. Three adult retinae we re examined which had mean axonal diameters of 1.18 +/- 0.86 (n = 5553 ), 1.12 g 0.79 (n = 7265), and 1.47 +/- 1.11 mu m (n = 10,867). Cumula tive histograms from several locations adjacent to the optic disc were unimodal (modal peaks: 0.6-0.8 mu m). This unimodal distribution, how ever, did not reflect the regional differences in axonal diameters fou nd throughout the retina. In many locations, especially those related to axons of the temporal retina, axon diameter distributions were clea rly bimodal or even trimodal (modal peaks: 0.6-0.8, 1.4-2.1, and 3.3 m u m). Measurements from one retina indicated that the mean diameters o f axons arising from the area centralis and visual streak (0.94 +/- 0. 63 and 0.98 +/- 0.68, respectively) were not significantly different f rom each other; however, when compared to other areas around the optic disc, the percentage of fibers with diameters between 1.5-2.0 mu m wa s 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 less than or equal to 0.05) and contained a higher percentage of medium-sized axons and fewer small axons. In each of the three retinae the proportions s mall, medium, and large axons were respectively gamma: 46; 47; 48, bet a: 50; 49; 48, and alpha: 4; 4; 4; regional differences in the proport ions of each axonal class are compared to previously published ganglio n cell density maps. Differences between axonal bundles within each sa mple location were not significantly different; however, in one retina axons in the scleral half of the fiber layer were significantly large r (P less than or equal to 0.01) than axons in the vitreal half of the nerve fiber layer adjacent to the optic disc. When compared to the ax onal diameter distributions found within the optic nerve (Cottee et al ., 1991) and optic tract (Reese et al., 1998), our data indicates that the diameter of retinal axons may increase by up to 30% along the len gth of the visual pathway.