Aging changes of the rhesus monkey optic nerve.

The authors counted axons in one optic nerve from each of 28 juvenile and aged rhesus monkeys using an automated image analysis system. All nerves were fixed immediately after death and treated identically. Animals ranged in age from 1 1/2 to 29 yr, which correlates with a human age range of 4 1/2-87 yr. Mean axonal number for all specimens was 1,117,859. Large variations in axonal numbers were found, even within a subgroup of ten animals aged 1 1/2-2 yr. Although young nerves had generally more fibers than those from old animals, this difference was not quite statistically significant, translating into a yearly loss of 4319 fibers (0.45% of the total). Age did not have any significant effect on mean axonal diameter.