Living fibre optics light up our eyes

4 May 2007

What's the secret to picture-perfect vision? Scientists now have the answer - they've discovered that humans have 'living optical fibres' that transmit light to the sensitive cells at the back of the eye. And copying nature could bring a new wave of 'intelligent' fibre optics.

Antenna looks into it...

Image: Stock.XCHNG/Henk L

The strangest thing about the human eye is that the layers of cells that line the back of it - the retina - are in the wrong order. Light has to travel through tiers of tissue to get to the sensitive cells that pick up the information.
In theory this should rule out pin-sharp sight, but now scientists have discovered why it doesn't. They've found that special cells in the retina called 'Muller cells' act in the same way as optical fibres to collect and channel light through our eyes.

This is an artist's impression of the retina. It shows a light beam passing through the many layers to the photoreceptor cells below.

Image: Jens Grosche

'All the nerve cells that transmit information from the eye to the brain are sitting in front of these light-sensitive (photoreceptor) cells,' says lead researcher Andreas Reichenbach. 'Usually we'd expect any light that is transmitted through so many layers to be reflected and distorted in the process.'
'So we wondered if there were any structures in the eye that helped get round this problem. Muller cells are the only cells running through the whole retina to the photoreceptor cells. Plus, these cells are long and stretched out like optic fibres, so we decided to investigate further.'

Andreas Reichenbach, Leipzig University, Germany.

Image: Leipzig University

Testing the theory out...
'We took parts of a living retina and looked at them under the microscope. We saw that in most of the retina the light was being reflected back, but there were these dark holes scattered through it like a Swiss cheese, where the light was actually passing through the retina,' Andreas explains.

This photo shows a cross section through the retina. The areas in red are the Muller cells that transmit light to the bright green photoreceptor cells. The duller green areas are the layers of nerve cells that would otherwise get in the way.

Image: Andreas Reichenbach

The scientists built up a picture of the eye and discovered that the holes were in fact tubes, running all the way through the retina. By using a dye that only Muller cells would pick up, the researchers showed that the tubes matched the exact shape and size of these cells.
But this was just the first step in proving their theory. 'At first, no-one believed us so we had to go back and do more experiments,' says Andreas.
'By conducting tests with lasers we finally proved that individual Muller cells could act like living optical fibres to transmit light to the photoreceptor cells.'

The team used a clever trick with lasers to trap a single Muller cell in the middle of two fibres and beam light through. They proved that more light was transmitted from one fibre to the other when the Muller cell was in-between.

Image: Stock.XCHNG/interact images

'Andreas and his team have uncovered a completely new function of this specialised cell that no-one had thought of before, which is really exciting,' says Frank Kirchhoff, a brain expert at the Max Planck Institute in Germany.
'Muller cells are not something that necessarily helps us on a sunny day when there's lots of light hitting the eye, but they are particularly important for helping us see at night when light levels are low.'

Frank Kirchhoff, Max Planck Institute for Experimental Medicine, Germany.

Image: Max Planck Institute

The scientists think their discovery could help engineers design the next generation of fibreoptic technology. 'Because Muller cells are funnel-shaped they can capture more light while only taking up a small area of the retina, leaving space for all the neurons that process the information,' Andreas explains.
By using the eye's design, engineers could fit other kit such as computer circuits into fibreoptic bundles, creating tiny 'intelligent' sensors. These could be used to look inside our bodies or even be fitted into army spying kit.

Image: Stock.XCHNG/Pawel K.

Frank agrees these findings could have important applications. 'What's really interesting is that these are long, soft, flexible cells. Mimicking such a structure could be useful for all sorts of detection devices and may even be used to make light-sensing materials we could wear.'
'But above all, these findings explain why we can see so well, even with our "inverted" eye - that's what is really astonishing,' says Andreas. 'What nature invented millions of years ago we only invented a few years ago!'