Reduction of phagosomes in the vitiligo (C57BL/6-mi(vit)/mi(vit)) mouse model of retinal degeneration

Purpose. The vitiligo (C57BL/6-mi(vit)/mi(vit)) mouse has a slowly progressing retinal degeneration, in which photoreceptor cell nuclei are gradually lost and the retinal pigment epithelium (RPE) is unevenly pigmented. The purpose of the present study was to assess the phagocytic ability of the RPE in the vitiligo mouse by determining whether and when a phagocytic burst occurs in affected mice and whether the number of phagosomes varies between control and affected animals. Methods. Eyes of control and vitiligo mice 4 to 20 weeks of age were embedded in Spurr. Thin sections were cut and examined by electron microscopy to confirm the presence of phagosomes, particularly in the affected animals. Thick (1 μm) sections were cut, and quantitative morphometry was performed at the light microscope level. The length of RPE was determined, and phagosomes were counted in RPE cytoplasmic and microvillous areas. Data were expressed as phagosomes per 1000 μm. Results. The vitiligo mouse has a peak phagocytic episode approximately 2 hours after light onset. The number of phagosomes in 4-week- old affected mice was significantly less than that in controls (13 phagosomes per 1000 μm compared to 30 phagosomes per 1000 μm). By week 8, the number was reduced to approximately 5 per 1000 μm. Phagosome number was not reduced further between weeks 8 and 20 in the affected animal. Macrophage-like cells containing pigment granules and phagosomes were observed in the subretinal space in areas where the rod outer segments had been separated from the RPE. Conclusions. The vitiligo mouse RPE contains phagosomes, but there are significantly fewer than in controls. It is not known whether a defect in RPE phagocytosis is the direct cause of the retinal defect in this model.