Editorial: The eye was evolution鈥檚 great invention
IT LOOKS wrong, but the strange, 鈥渂ackwards鈥� 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鈥檚 peculiarities, they help filter and focus light, making images clearer and keeping colours sharp.
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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鈥檚 biggest 鈥渕istakes鈥�.
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 for the rods and cones. These M眉ller 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 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 M眉ller 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 鈥渘oise鈥� that has already been reflected multiple times within the eye. The simulations showed that the M眉ller 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 M眉ller cell was unlikely to be taken up by a neighbour, because the surrounding nerve cells help disperse it. What鈥檚 more, the intrinsic optical properties of M眉ller cells seemed to be tuned to visible light, leaking wavelengths outside and on the edges of the visible spectrum to a greater extent.
鈥淢眉ller cells seem tuned to visible light, leaking other wavelengths to a greater extent鈥�
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 M眉ller cells鈥� wide tops allow them to 鈥渃ollect鈥� any separated colours and refocus them onto the same cone cell, ensuring that all the colours from an image are in focus (see diagram).
鈥淚t suggests that light-coupling by M眉ller cells is a crucial event that contributes to vision as we know it,鈥� says , a neurophysicist at the University of Cambridge and co-author of the 2007 study. 鈥淭his work nicely complements our experimental data.鈥�
However, , a biologist at Brown University in Providence, Rhode Island cautions that this doesn鈥檛 mean that the backwards retina itself helps us to see. Rather, it emphasises the extent to which evolution has coped with the flawed layout. 鈥淭he shape, orientation and structure of the M眉ller cells help the retina to overcome one of the principal shortcomings of its inside-out wiring,鈥� says Miller.
The new understanding of the role of M眉ller cells might find applications in more successful eye transplants and better camera designs, says Ribak.
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Editorial: The eye was evolution鈥檚 great invention
