Living Optical Elements in the Vertebrate Retina

While cells are mostly transparent they are phase objects that differ in shape and refractive index. Any image that is projected through layers of cells will normally be distorted by refraction, reflection, and scattering. Strangely, the retina of the vertebrate eye is inverted with respect to its optical function and light must pass through several tissue layers before reaching the light-sensitive photoreceptor cells, with each photon having a chance of being scattered. Here we report how nature has optimized this apparently unfavourable situation. We investigated the optical properties of retinal tissue and individual Muller cells, which are radial glial cells spanning the entire thickness of the retina. Using confocal microscopy, quantitative refractometry, and a modified fiber-based dual-beam laser trap, we found that these cells act as optical fibers and guide light, which would otherwise be scattered, from the retinal surface to the photoreceptor cells. Their parallel arrangement in the retina is reminiscent of fiber-optic plates used for low-distortion image transfer. Behind the Muller cells, there seems to be a specific adaptation of the rod photoreceptor nuclei for improved light transmission through the outer nuclear layer of nocturnal animals. These nuclei have an inverted chromatin structure that turns them into micro-lenses channeling the light through the ONL. These findings ascribe a new function to glial cells, demonstrate the first nuclear adaptation for an optical function, and shed new light on the inverted retina as an optical system.