SD-OCT was performed using a Bioptigen Spectral Domain Ophthalmic Imaging System (Bioptigen, Inc., Durham, NC, USA) as previously described.28 (link) The system has a platform designed for easy orientation and aligning of mice for retinal imaging, and provides a high resolution of 2 μm. The mice were anesthetized by intraperitoneal injection with a mixed solution of ketamine and xylazine. The pupils were dilated with 1% tropicamide eye drops prior to imaging. Radial volume scan (centered on the optic disc, consisting of 100 B-scans) was acquired using image analysis software (InVivoVue Clinic; Bioptigen, Durham, NC, USA). Outer nuclear layer (ONL) thickness is an indicator of photoreceptor survival, and measurements were assessed using ImageJ (National Institutes of Health) software29 (link) on four selected scans (scan numbers 1, 26, 51, 76 represent 0°, 45°, 90°, and 135°, respectively, in en face images).30 (link) The lines defining ONL (between the outer plexiform layer to the outer limiting membrane) were drawn in ImageJ (2 μm/pixel), one on each side of the optic disc, 300 μm from the optic disc. The measurements of ONL thickness were performed by experienced investigators in a masked fashion.