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 pro gressing retinal degeneration, in which photoreceptor cell nuclei are gradually lost and the retinal pigment epithelium (RPE) is unevenly pi gmented. The purpose of the present study was to assess the phagocytic ability of the RPE in the vitiligo mouse by determining whether and w hen a phagocytic burst occurs in affected mice and whether the number of phagosomes varies between control and affected animals. Methods. Ey es 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 c onfirm the presence of phagosomes, particularly in the affected animal s. Thick (1 mu m) sections were cut, and quantitative morphometry was performed at the light microscope level. The length of RPE was determi ned, and phagosomes were counted in RPE cytoplasmic and microvillous a reas. Data were expressed as phagosomes per 1000 mu m. Results. The vi tiligo 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 mu m compared to 30 phagosomes per 1000 mu m). By week 8, the number was r educed to approximately 5 per 1000 mu m. Phagosome number was not redu ced 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 s eparated from the RPE. Conclusions. The vitiligo mouse RPE contains ph agosomes, but there are significantly fewer than in controls. It is no t known whether a defect in RPE phagocytosis is the direct cause of th e retinal defect in this model.