The image above is of the living cone photoreceptors at the back of my eye. The photoreceptors are the cell types in our eyes that detect light and then transduce that signal into neural impulses that are computed by the retina before sending that information on to our brains. The image below is the retinal pigment epithelium, a single layer of cells in your eye that supports the photoreceptors and keeps them alive and healthy.
Imaging these structures in the living eye is a triumph of applied science, using adaptive optics, a set of technologies borrowed from the astronomy, remote sensing and defense communities. These technologies are still in development by researchers like my friend Joe Carroll, and will become more important to track the progression of retinal disease that causes blindness. There are progression points of retinal disease processes that we have been studying that may be more amenable to intervention than others and we have to be able to identify those points in people at various stages of their disease progression.