AIM: To observe the early retinal degeneration and activation of microglia in C57BL/6N(Crb1rd8/rd8)mice.METHODS:Totally 15 male SPF C57BL/6N mice and 15 male SPF C57BL/6J mice were raised normally, and fundus photography examinations were performed by Micron-Ⅲ at the time of 0, 4, 8, 12 wk of enrollment to calculate the number and area of retinopathy. At the end of experiment, all mice were sacrificed and the right eyeballs were removed to prepare retinal tissue slices. After HE staining, the retinal tissue morphology was observed under optical microscope while the location and level of CX3CR1 expression were detected in immunohistochemical staining. The left eyeballs were removed to isolate retina, then Western-Blot was used to analyze the expression of CD86 and CD206 proteins in retina, and the concentration of IL-1β, IL-6, TNF-α, IL-4 and IL-10 in retina was detected by electrochemiluminescence.RESULTS:The result of fundus photography examinations showed that the number of retinopathy in the C57BL/6N significantly increased at 4, 8, and 12 wk, and there were differences in variations compared with the C57BL/6J at the same time point(all P<0.05). In the changes in area of retinopathy, there was a difference between two groups at 12 wk(P<0.05), but no difference in variations within groups(both P>0.05). HE staining of retinal tissue showed that the retinal structure of C57BL/6N mice was abnormal, with loose and disordered cell arrangement, and the photoreceptor layer was obviously protruding to the inner side of retina with a drusen-like protrusion. The retinal structure of C57BL/6J mice was clearer, with orderly cell arrangement and no obvious abnormality. Immunohistochemical results showed that CX3CR1 was highly expressed in ganglion cell layer, inner and outer plexiform layer, photoreceptor cell layer and lesion in the retina of C57BL/6N mice, with a mean density of 0.285±0.056 in C57BL/6N and 0.189±0.084 in C57BL/6J mice(P<0.05). The results of Western-Blot showed that the expression of CD86 and CD206 in retina of C57BL/6N increased compared with that in C57BL/6J to varying degrees, and the difference of CD86 was statistically significant(P<0.05). The results of cytokine detection showed that the level of IL-1β, TNF-α in C57BL/6N was significantly higher than that of C57BL/6J, while IL-10 was significantly lower(all P<0.05).CONCLUSION: The retinal degeneration of C57BL/6N(Crb1rd8/rd8)mice progressed slowly and gradually aggravated with age. The retinal structure of the lesion was disordered and accompanied by microglial infiltration dominated by M1 polarization.