Evolution put a notorious quirk in the vertebrate eye, placing the light-sensing cells on the back of the retina instead of the front. But evolution also seems to have found a high-tech work-around for this apparent mistake. Scientists now say that specialized cells transmit light through the retina’s layers of various cells by acting like optical fibers.
In vertebrate retinas, light has to cross up to one-fifth of a millimeter of connective and nerve cells before reaching light-sensing cells.
But a team in Germany has shown that some of the layers’ cells, shaped like funnels and called Müller cells, have a higher refractive index than the others. The scientists reached this conclusion by shining a laser through Müller cells taken from guinea pigs.
The higher refractive index would enable Müller cells to channel light with little loss, as optical fibers do. Light traveling along an optical fiber doesn’t escape laterally because it gets reflected at the boundary between a high-refractive-index core and a lower-index sheath.
Müller cells probably get their high refractive indexes from tight bundles of polymer fibers extending along their lengths, says team member Jochen Guck, now of the University of Cambridge in England. The findings appeared in the May 15 Proceedings of the National Academy of Sciences.
Barbara Finlay, a neurobiologist at Cornell University, says that the cells’ behavior should be checked at different wavelengths of light. But she says the new findings could be the kind of unexpected discovery that seems “perfectly obvious” in hindsight.