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W. A. Beltran, G. M. Acland, G. M. Aguirre; Morphologic Analysis of Retinal Disease in the Area Centralis of RPGR-ORF15 Mutant Dogs. Invest. Ophthalmol. Vis. Sci. 2007;48(13):3718.
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Mounting evidence suggests that central rod and cone visual deficits are a characteristic of RPGR-XLRP. Using a recently characterized dog model of early onset XLRP caused by a frameshift mutation in RPGR-ORF15, we examined retinal disease in the area centralis, the canine homolog of the human foveo-macular region.
Retinas from 10 affected dogs (2 weeks-19.9 weeks) were used. The tissues were fixed and embedded either in epoxy resin (5 retinas) or in OCT (5 retinas). Cytologic characteristics of the disease were examined on semithin plastic sections, and on H&E stained cryosections. Immunohistochemistry using a battery of cell-specific antibodies was used to examine photoreceptors (PRs) and inner retinal cells.
The region of highest ganglion cell density (2-3 rows) of the area centralis was easily located on retinal cross-sections through the temporal meridian. Within this region, a significant thinning of the ONL was observed as early as 2 weeks of age. This site (approx 200 µm in diameter) was located 3744 ± 306 µm temporal to the optic disc and characterized by a more severe decrease in rod density. Double rows of cones were frequently seen. A localized increase in S cone density was found. Preliminary IHC results do not suggest any major morphologic alterations of the inner retina at early ages.
Although pan-retinal PR degeneration and remodeling occurs in affected dogs, an early and more severe rod loss was found in a specific site of the area centralis. The early age at which these findings were seen suggests that rods do not develop normally in this area. This may lead to subsequent cone dysfunction and death. A similar mechanism could be responsible for central visual deficits in some human patients with RPGR-XLRP.
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