IT LOOKS wrong, but the strange, "backwards" structure of the vertebrate retina actually improves vision.
Certain cells act as optical fibres, and rather than being just a workaround to make up for the eye's peculiarities, they help filter and focus light, making images clearer and keeping colours sharp.
Although rods and cones are responsible for capturing light, they are in a curious position. Hidden at the base of the retina, they are covered by several layers of cells as well as the bed of nerves that carries visual information to the brain. One result is a blind spot in our visual field, leading the vertebrate retina to be listed among evolution's biggest "mistakes".
Light clearly gets through, however, and in 2007 researchers analysing the retinas of guinea pigs reported that the glial cells which nourish and physically support the bed of neurons also act as optical fibres for the rods and cones. These Müller cells are funnel-shaped, with wide tops that cover the surface of the retina and a long slender body that guides light to the receptors below.
Now Amichai Labin and Erez Ribak of the Technion-Israel Institute of Technology in Haifa have used data from human eye cells to model the workings of the retina. Their findings suggest that sending light via the Müller cells offers several advantages.
At least two types of light get inside the eye: light carrying image information, which comes directly through the pupil, and "noise" that has already been reflected multiple times within the eye. The simulations showed that the Müller cells transmit a greater proportion of the former to the rods and cones below, while the latter tends to leak out. This suggests the cells act as light filters, keeping images clear.
The researchers also found that light that had leaked out of one Müller cell was unlikely to be taken up by a neighbour, because the surrounding nerve cells help disperse it. What's more, the intrinsic optical properties of Müller cells seemed to be tuned to visible light, leaking wavelengths outside and on the edges of the visible spectrum to a greater extent.
The cells also seem to help keep colours in focus. Just as light separates in a prism, the lenses in our eyes separate different colours, causing some frequencies to be out of focus at the retina. The simulations showed that Müller cells' wide tops allow them to "collect" any separated colours and refocus them onto the same cone cell, ensuring that all the colours from an image are in focus (see diagram).
"It suggests that light-coupling by Müller cells is a crucial event that contributes to vision as we know it," says Kristian Franze, a neurophysicist at the University of Cambridge and co-author of the 2007 study. "This work nicely complements our experimental data."
However, Kenneth Miller, a biologist at Brown University in Providence, Rhode Island cautions that this doesn't mean that the backwards retina itself helps us to see. Rather, it emphasises the extent to which evolution has coped with the flawed layout. "The shape, orientation and structure of the Müller cells help the retina to overcome one of the principal shortcomings of its inside-out wiring," says Miller.
The new understanding of the role of Müller cells might find applications in more successful eye transplants and better camera designs, says Ribak.
Source: New Scientist