Natural Fiber Optic Plates

David L. Shenkenberg

A research team has discovered that cells in the retina of vertebrates function like fiber optic plates, yet the ways in which they differ from man-made materials could provide clues for building superior optical fibers and fiber optic plates. For example, these cells not only route light but also collect it, while occupying less space than artificial fiber optic cables.

Researchers demonstrated that cells of the vertebrate retina called Müller cells are natural optical fibers that guide light to photoreceptors of the eye, as shown in this artist’s impression. These cells could inspire new designs for man-made optical fibers and fiber optic plates. Courtesy of Jens Grosche.

The team was affiliated with Universität Leipzig and with Universität Göttingen, both in Germany, with Universidade Central de Caribe in Bayamon, Puerto Rico, and with the University of Cambridge in the UK.

The researchers suspected that certain cells of the retina, called Müller cells, could be analogous to man-made fiber optic plates because these cells resemble fibers. In fact, their original name was “radial fibers of Müller.” Furthermore, they have a regular pattern of parallel structures like blades of grass on a lawn, and they look like a plate because they span the entire thickness of tissue.

Initial transmission and reflection microscopy measurements of dissected eyes showed that some structures in the retina relay light better than others, and these structures resembled Müller cells. To prove that these structures are Müller cells, the scientists showed that a Müller cell could propagate light as would a man-made fiber. They used a Coherent argon-ion laser as the visible light source, and they measured light propagation in terms of optical power, employing a Coherent power meter.

In a past experiment, the researchers measured this text (left) with a commercial fiber optic plate. They imaged the text on the right using a guinea pig retina, demonstrating that both can show the same information. The dotted line indicates the retinal margin. Courtesy of Kristian Franze.

After coupling the laser and power meter to respective artificial fibers, they aligned the Müller cell like a bridge between these man-made fibers. The researchers observed that the Müller cell propagated the light from the laser to the power meter. In fact, the optical power received by the power meter was more than twice as great as it was in the absence of the Müller cell.

But Müller cells are different from artificial optical fibers that have a core and a cladding, each with homogeneous refractive indices. The researchers measured the refractive indices along the length of a Müller cell by focusing polarized light on the cells, and the cells changed the angle of polarization based on their refractive index. The researchers found that the mean refractive index of Müller cells was significantly higher than that of the surrounding neurons, and that the refractive index decreased toward the end of the cell nearest to where light enters.

Additionally, the diameter was known to change across the length of the cell, which is not the case with artificial optical fibers. Even though the refractive index and diameter change, Müller cells can transmit light because the waveguide characteristic frequency stays constant, the researchers noted. They also observed that the Müller cells do not propagate light via total internal reflection fluorescence, in contrast to man-made optical fibers.

Above all the things interesting to optical engineers, the researchers said that Müller cells are much thinner than artificial optical fibers, occupying less than 20 percent of the retina’s cross-sectional area. This leaves room for retinal neurons that preprocess the image information before it reaches the body’s central computer — the brain.

“If it is possible to generate optic fiber plates with such ‘trumpet shaped’ fibers, the remaining volume could be used to insert small computing elements to generate an ‘intelligent’ sensor,” said principal investigator Andreas Reichenbach.

PNAS, May 7, 2007, doi: 10.1073/pnas.0611180104.