A case of focal retinal atrophy in a cynomolgus monkey

Abstract

Cynomolgus monkeys are nonhuman primates that are commonly used in nonclinical safety research. In this species, there have only been a few reports of spontaneous findings that were identified on ophthalmological or histopathological examination. Herein, we report a case of focal retinal atrophy in a 2-year-old male cynomolgus monkey. This animal was being used in a safety study of a candidate drug compound. During an ophthalmic examination conducted prior to the study, a focal retinal discoloration on the temporal side of the macula was observed in the right eye of the monkey. An ophthalmological examination approximately 3 months later revealed no progression of the lesion, with no changes in its color or area. After the ophthalmological examination, this eyeball was removed at necropsy and fixed. The lesion was a dark, depressed area of approximately 5 mm in size on gross examination. Histopathological examination revealed marked full-thickness thinning of the neural retina, which corresponded to the area of discoloration. Only ganglion cells and cells in the inner nuclear layer in the neural retina were identified. The retinal pigment epithelium (RPE) arrangement was slightly irregular but continuous, and there was no defect or degeneration. No abnormalities were found in the choroid. Immunohistochemical examination using anti-glial fibrillary acidic protein (GFAP) antibody revealed that most of the cells in the inner nuclear layer in the lesion were activated Müller cells. The differential diagnoses included atrophy as acquired and coloboma and hypoplasia as congenital are considered, but the published histopathological features of the latter two, such as hypoplasia or defects in RPE cells and the choroid, were not consistent with those of the present lesion. Reactive gliosis involving Müller cells has been reported to be present in pathologies such as mechanical injury, retinal ischemia, and glaucoma. In the present case, the presence of many GFAP-positive Müller cells was considered to be glial scar formation after an injury. Therefore, we finally diagnosed this lesion as focal retinal atrophy.