Physics of Medicine

Research in Physics of Medicine

Guiding the Light

Pioneering research is shining new light on our understanding of the way we see the world. Optical fibres have now been found to exist in vertebrate eyes, channelling light down their length and delivering it without distortion straight to the cells that 'see'.

Incredibly sophisticated in structure and function, the construction of the retina has puzzled researchers ever since the finer structures of the eye were first resolved over 150 years ago: the retina is built the 'wrong' way around. The cells responsible for light sensing are sited at the back of the eye, furthest from the incoming light. An advanced understanding of the 'inverted retina' has now been revealed by Dr Jochen Guck, newly arrived at the Cavendish Laboratory, while working with a team of scientists at the University of Leipzig, Germany. The results of this study were published in PNAS (Franze et al., Müller cells are Living Optical Fibers in the Vertebrate Retina. Proc. Natl. Acad. Sci. U.S.A. 104(20):8287-8292, 2007). In 2008 this article was recognized with a Cozzarelli Prize for scientific excellence and originality.

Because of its inverted structure, light has to pass through several cells in the retina before it reaches the photoreceptor cells that capture the image and transmit it to the brain. How does this happen without the light being scattered and distorted?

Light shining
through Müller cells

Light shining through Müller cells
Acknowledgement: Dr Jens Grosche, Universität Leipzig

An understanding of this enigma has become possible with the invention of a special dual-beam laser trap or 'Optical Stretcher' by Dr Guck and colleagues, in which physical, light-transmitting properties can be visualised and measured at the level of a single cell. Using this tool, the researchers discovered that the answer to the mystery lies with specialised, elongated cells known as Müller cells, which span the retina and have an amazingly high refractive index compared with their surroundings. This difference in refractive index effectively means the light 'bounces' along the cell and barely leaks.

The ground-breaking studies showed that the Müller cells essentially act as a field of miniature optic fibres - lined up in parallel in the direction of the light and traversing the whole retina. They trap the light, guide it down their length, and deliver it to the photoreceptors waiting to receive the stimulus. This is just one example of the exciting research possibilities using light to investigate the mechanical and optical properties of living cells and tissues as part of the new Cavendish Laboratory Initiative in the Physics of Medicine.

For more information, please contact Dr Jochen Guck