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Benjamin S Sajdak, Christopher S Langlo, Yusufu N B Sulai, Robert F Cooper, Dana K Merriman, Joseph Carroll, Alfredo Dubra, Advanced Ocular Imaging Program; Non-invasive adaptive optics imaging of the ground squirrel retina. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):4099.
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© ARVO (1962-2015); The Authors (2016-present)
Ground squirrels are an important model for studying visual processing, retinal circuitry, and cone photoreceptor function. Using a custom adaptive optics scanning light ophthalmoscope (AOSLO), we sought to noninvasively image the microscopic structure of the ground squirrel retina.
Four 13-lined ground squirrels (Ictidomys tridecemlineatus) were anesthetized with isoflurane (5% induction, 2-3% maintenance) and placed on a heated stage. Pupils were dilated with one drop of 2.5% phenylephrine and one drop of 1% tropicamide. Saline drops were applied as needed to maintain corneal hydration. Reflectance, confocal and split-detection AOSLO imaging through a 4 mm diameter pupil was performed with near infrared light (790 nm). The pixel size in microns was calculated using a Ronchi ruling placed in the back focal plane of a 19 mm focal length model eye, and then scaled according to the axial length of each animal, measured with an ultrasound A-scan (OTI-Scan 1000). Photoreceptor density and Voronoi geometry were calculated at known retinal locations using cell coordinates obtained by a semi-automated cell counting algorithm.
High-resolution images of the photoreceptor mosaic, nerve fiber layer, and retinal vasculature (Fig.1) were obtained with a high success rate (100%, n = 4). No lens opacities were observed during any of the imaging sessions, which lasted up to 1 hour. Photoreceptor density values are consistent with previously published histological data of a congener species (Long and Fisher, J Comp Neurol., 1983; Kryger et al., Vis Neurosci., 1998), ranging from 25,600-78,400 cells/mm2. The mosaic displayed a fairly triangular packing geometry, with 42-56% of the cells having six-sided Voronoi domains around the horizontal streak, and 59-69% in more peripheral locations.
The photoreceptor mosaic, retinal capillaries and microscopic detail on the nerve fiber layer inner surface of the ground squirrel were visualized with AOSLO. The image quality and success rate appears to be better than current AOSLO efforts on mice and rats. Thus, when combined with the non-invasive high resolution imaging afforded through AOSLO, the ground squirrel could serve as a useful model to aid drug discovery and testing through longitudinal imaging on the cellular scale.
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