Quantitative and morphological changes of the choroid vasculature in RCS rats and their congenic controls

In Royal College of Surgeons (RCS) rats the outer retina degenerates owing to a phagocytic defect of the retinal pigmented epithelium (RPE). To analyse whether the choriocapillaris also undergoes degenerative changes, corrosion casts of choroidal vessels of ten RCS rats and 12 age-matched controls of different age groups were investigated by scanning electron microscopy. The pattern of the ciliary arteries was analysed qualitatively and the areas covered by capillaries in the different quadrants and regions of the choroid were evaluated quantitatively. In both RCS rats and their congenic controls the choroid was supplied by only three ciliary arteries. A separate artery supplying the upper quadrant in other rat strains was lacking. The quantitative evaluation revealed that in controls the capillary density decreased from the central to the peripheral regions. In addition there were slightly fewer capillaries in the upper temporal quadrant compared to other quadrants. In RCS rats older than 6 months, all parts of the choroid displayed a decrease in the number of capillaries of about 10-15% compared to the controls. Most prominent loss of capillaries was found in the upper temporal quadrant. Electron-microscopical investigation of the upper temporal quadrant showed that loss of choriocapillaris was directly correlated with degeneration of the adjacent retinal pigmented epithelium (RPE).