Hiding in the back of your eye are stem cells from the central nervous system, scientists have discovered. The stem cells in question were found in a special layer of cells in the eye called the retinal pigment epithelium, or RPE. Just one cell thick, the layer lies underneath the retina, the eye’s light sensor. The RPE keeps the retina alive and functioning. In diseases like macular degeneration, which affects 10 million Americans, the RPE fails and the retina dies.