they're really essential for photoreceptor health and for photoreceptor survival. and several blinding eye diseases involve -- and their treatments -- involve these pigment epithelial cells in the retina. we'll hear about these cells. they'll feature really quite large in our conversation. >> charlie: that's enormously helpful. thank you very much for the huge understanding of how we see and how important the brain is and how important the pathways are. i want to come now to jean and talk about -- we mentioned gene therapy and stem cell therapy and other things. how can gene therapy be helpful? >> well, carla has set the stage for this perfectly because she's explained how the pigment epithelium provides nerve function for the photoreceptors, providing nutrients to the cells, taking away waste products, and the two cell types are interdependent so that, if there's a problem in function of one particular gene in one of those cells, it affects the other cell secondarily. so in the next image is an illustration of one particular example where a mutation can cause dise

and those are called the photoreceptors. there are two kinds of photoreceptors. the rods and cones. the cones are crucial for high resolution vision and they are in the center. the rods are the photoreceptors in the peripheral retina and those are really essential for high sensitivity vision. the next step is that the information from the photoreceptors is passed along to the bipolar cells. and from the bipolar cells, the information is passed along to the ganglion cells. they are the nerve cells that spend their long fibers into the optic nerve and the optic track. one important point i want to make, there is another critical type in the right now. it is called the pigment. its cells provide crucial support and nutrients to the photoreceptors. they are really essential for the health and survival. several eye diseases involve these cells in the retina. we will hear about them. i think they will feature quite large in our conversation. >> that is helpful. understanding how we see. and how important the brain is and the pathways are. ♪ >> i want to come to jean, we mentioned stem cel