2.5. Optic nerve axon count method
To quantify RGC loss, we quantified axon loss in optic nerve cross-sections using a validated sampling method (Levkovitch-Verbin et al., 2002). The optic nerves were fixed either by perfusion or immersion as described above, then post-fixed in 1% osmium tetroxide, dehydrated in alcohol, and stained with 1% uranyl acetate in 100% ethanol for 1 h. They were then embedded in epoxy resin, sectioned at 1 mm, and stained with Toluidine Blue. The cross- sections were digitally imaged (Cool Snap camera, Metamorph Image Analysis software; Molecular Devices, Downington, PA) at 10 to assess optic nerve area. Five higher power images (40 mm square, 100 ) were used to measure axon density by observers masked to the protocol used for the eye. To ensure that non-axonal elements were not mistaken for axons, the high power images were individually viewed and edited to include only axon profiles. The mean axon density was multiplied by the nerve area for each sample to give total axon number. In order to yield percent axon loss, we compared this estimated total axon number for each sample against the mean of control or fellow eye axon counts.