Retinal rods make light work of night vision

Scientists have uncovered a previously overlooked trick used by nocturnal animals to allow them to see in the dark.

Writing in this week's Cell, Boris Joffe and his colleagues at Ludwig-Maximilians University Munich have found that animals active at night use their own DNA as miniature lenses to channel light through their rod cells.  This is to overcome the problem is that the retina, the light-sensitive part of the eye, is effectively "inside out".  In other words, the light-detecting rod and cone cells are positioned closest to the back of the eye where they can pick up sufficient oxygen to meet their needs.  This means that to reach them light first has to filter through several layers of supporting cells positioned closer to the front of the eye and whose job it is to transmit the information from the rods and cones back to the brain.  But this leaves noctural animals with a problem because they must respond to light levels a million times lower than those available during the day.

The solution, it turns out, was found lurking in their DNA, which is, ironically, also inside out.  DNA is normally tightly packed inside the nucleus of a cell in such a way that the dense, highly-active parts of the genome are clustered centrally and the inactive (junk) is scattered around the outside.  In the rod cells of night-active animals, however, this situation is reversed, but no one could originally work out why.

Now, by studying rod cells under a microscope and building a computer model of their observations, the German team have found that this arrangement of DNA behaves like a lens, helping to focus light into the light-sensitive tips of the rod cells. When the team initially proposed the idea some were hard to convince, "people laughed at first," says Joffe.