Genetic diseases like retinitis
pigmentosa destroy the photosensitive cells of the eye, the photoreceptors, but often leave intact the other cells in the retina: the bipolar cells that the photoreceptors normally talk to, and the ganglion cells that are the retina's output to the brain.
But someday all of them could offer a valid treatment for retinitis
pigmentosa as well as for age - related macular degeneration, which gradually
destroys photoreceptors in the center of the retina and is the leading cause of blindness in adults over age 55.