The Ancient Part of Your Eye

The internal biological clock, which keeps most organisms on a 24-hour cycle, is set by changes in levels of light.  Three years ago, researchers from Brown University discovered branch-like eye cells in mammals that were responsible for telling the clock whether it was day or night.

Now, the scientists have uncovered some of the mechanisms in these light-sensors, called intrinsically photosensitive retinal ganglion cells, or ipRGCs.

The ipRGCs reside deeper in the retina than the rods and cones - the millions of eye cells through which we see the outside world.  Only numbering between 1,000 and 2,000, ipRGCs gauge the overall light intensity and relay this information to a tiny region in the brain that controls the body clock. 

This direct link explains how some blind people can still have the same daily internal rhythm as a sighted person.

The research team, led by neuroscientist David Berson, has discovered a chain of chemical reactions in ipRGCs, set off by light-absorption in the protein melanopsin.  This process is similar to the way that the eyes work in fruit flies and squid.

"The results may well tell us that this is an extremely ancient system in terms of evolution," Berson said. "We may have a bit of the invertebrate in our eyes."

 -- Michael Schirber

Credit: Brown University