Frequent non-intact axons and extensive extracellular debris in the optic nerve head at very early stages of non-human primate experimental glaucoma

Abstract

Purpose : To examine non-intact axons within the optic nerve head (ONH) of Non-Human Primate (NHP) early experimental glaucoma (EG) eyes, and determine whether there is any relationship between electron lucent extracellular structures and the location of axon ends.

Methods : Two 200x200x50 um serial block face scanning electron microcopy (SBEM) ONH volumes obtained from 2 regions of a NHP eye at early stages of degeneration were compared to the corresponding regions in its contralateral control eye and 2 regions of a second control ONH. For approximately 2-5% of axons were chosen at random on the vitreal side of each volume their continuity within the volume was established by manual tracing. Each axon that ended within the volume was termed “non-intact”, and the distance from its endpoint to the nearest pocket of extracellular electron lucent material was measured.

Results : Among all EG and Control eye volumes, of 1140 axons chosen for tracing, over half were uninformative as they exited the volume before reaching the chiasmal side, 440 were intact through the volume, and 11 non-intact axons were identified. All 11 non-intact axons were in one of the two EG eye volumes. The ratio of non-intact to intact axons was 2/98 for a the EG eye volume with 8% estimated axon loss and 9/132 for the EG volume region with 15% estimated axon loss. The non-intact axon ends were found both in unmyelinated and myelinated regions, and often associated with phagocytic cells and/or large “pockets” of extracellular electron lucent material. Several non-intact axons manifested acute turns near their ends. Reconstructions of nearby electron lucent extracellular pockets showed them to be highly heterogenous in size and shape, up to 290 um³ in volume, and not recognizable as any known structure. The mean distance from non-intact axon ends to the nearest electron lucent pocket was 10.8 +/- 7.8 um (SD), which was not significantly different from distances from points chosen at random within the same blocks to the nearest electron lucent pocket.

Conclusions : EG eye ONH regions with no evidence of pre-sacrifice OCT change and 8-15% post-mortem axon loss contain evidence of active axonal degeneration, with 2-6% axons ending within the block, and frequent axon bundle-sized pockets of extracellular debris. No clear association between axon ends and electron lucent pockets was evident.

This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.