Axon morphology at the lamina cribrosa in monkey eyes

The eyes of 8 monkeys (Aotus trivirgatus) were studied. The mean cross-section area and the least diameter of axon cylinders were calculated from measurements made by computer assisted planimetry of electron photomicrographs of sections through the optic nerve head at the level of the lamina cribrosa. The density of intrabundle connective tissue and glial cell elements in nerve fiber bundles was also calculated. The mean cross-section area and minimum diameter of axons in the temporal part were less than in the nasal part of the nerve. The values for axons in the superior and inferior parts of the nerve were intermediate. A similar pattern of increasing dimensions was seen in axons from the more axial nerve compared to neurons in the more circumferential nerve sectors. The density of the intrabundle, nonaxonal tissue elements did not differ significantly across the nerve. Although axon dimensions may play some role in defining the vulnerability of neuronal tissue to a pressure insult, the results of this anatomic investigation do not support the hypothesis that differences in axonal distribution by size across the nerve section define the regional vulnerability of the nerve head to elevated intraocular pressure.