Cells in eye not there just for looks
Müller cells guide light to receptors in the back of the eye
David L. Shenkenberg
Until recently, it was believed that light travels to receptors in the back of the eye by uncontrolled scattering, but researchers from Universität Leipzig and Universität Göttingen, both in Germany, and from Universidade Central de Caribe in Bayamon, Puerto Rico, and from the University of Cambridge in the UK, now have discovered that Müller cells guide light to those receptors.
For more than a century, it has been known that Müller cells are present in the eye, but they have been ignored until recently, according to principal investigator Andreas Reichenbach. It also was known that they are arranged in a regular pattern of parallel fibers that span the entire retina, from the vitreous body to the photoreceptors (Figure 1). From this information, the researchers hypothesized that Müller cells can conduct light; thus, they are analogous to man-made optical fibers.
Figure 1. Researchers have discovered that light travels through Müller cells — like fiber optic cables — to photoreceptors in the back of the eye. The image on the left shows light entering the eye, while the middle image depicts light traveling through the eye, and the image on the right portrays light traveling through the cells.
However, it was doubted that they could direct light because they are more complicated than artificial fibers. For example, they are not the same size across their length, and they have side branches that extend from the main tube. Furthermore, they are not specialized for guiding light because they also have biochemical purposes.
In all of the researchers’ experiments, they studied dissected eyes from several animals, including humans, to test whether the light-guiding property of Müller cells applies to vertebrates in general.
In initial experiments, they used a Carl Zeiss confocal microscope to make transmission measurements of dissected eyes. They inserted a man-made optical fiber into the eye cup to simulate physiological lighting. As a result, dots of light could be seen through the microscope, showing that some structures in the retina relay light better than others, but it was yet to be proved that these were Müller cells.
To replicate the pattern of light dots, the scientists used the microscope in reflection mode, which should produce the negative of the transmission-mode image. Indeed, the reflection measurements yielded the opposite image, which looked “like holes in Swiss cheese,” Reichenbach said. The holes had the spatial distribution of Müller cells, as did the light dots observed in transmission mode, suggesting that the structures that caused these holes were Müller cells. Because reflection measurements can be done in living animals, the researchers took reflection measurements in living guinea pigs, which yielded the same result.
To further show that the structures were Müller cells, they also compared lateral sections of eye tissue examined in reflection mode with those sections when labeled with a fluorophore that emits green light (Figure 2). The structures closely matched, indicating that the reflection-mode images of the light-conducting structures were of the cells.
Figure 2. This reflection-mode microscopy image (left) shows structures that are dark, demonstrating that they conduct light. The structures are similar to these green fluorescently labeled Müller cells (right), indicating that the light-conducting structures are the cells. Reprinted with permission of PNAS.
Natural optical fibers
To test the hypothesis that Müller cells are natural optical fibers, the researchers performed refractometry on the cells. They focused polarized light on them, and their cylindricity altered the angle of polarization. They measured the change in the angle with a phase microscope from Lomo plc of St. Petersburg, Russia. The diameters of the cells and the angle of light were measured, and the researchers plugged the results into an equation.
They found that Müller cells are more complex than artificial optical fibers but that the cells still comply with physical theory governing waveguides. For one thing, their mean refractive index is significantly higher than that of neighboring tissue elements but lower than that of most artificial optical fibers. Additionally, both the refractive index and the diameter change across the length of the cell, whereas neither variable changes throughout artificial optical fibers. Although the Müller cells do not have all of the characteristics of optical fibers, the researchers showed that they can transmit light because the waveguide characteristic frequency stays constant, and this parameter is what defines a waveguide, as the name implies.
Because the Müller cells are not entirely the same as optical fibers, the researchers did an experiment to prove that the cells are optical fibers. They showed that a cell can propagate light the same as a man-made fiber. They used a Coherent argon-ion laser as the visible light source and measured light propagation with a Coherent power meter. After coupling the laser and power meter to respective artificial fibers, they aligned the cell like a bridge between the man-made fibers with a dual-beam laser trap (Figure 3). The trap consisted of a 1064-nm fiber laser from IPG Photonics of Burbach, Germany, coupled to two singlemode fibers.
Figure 3. To prove that Müller cells are fiber optic cables, the researchers showed that visible light from a fiber-coupled laser travels through a cell to a fiber-coupled optical power meter, rather than scattering. Reprinted with permission of PNAS.
The investigators observed that the Müller cell propagated the light from the laser to the power meter. In fact, the optical power received by the meter was more than twice as great as it was in the absence of the cell. The researchers also misaligned the two fibers — so that hardly any light from the first fiber would hit the second one — and found that the power increased even more dramatically when the cell bridged the distance between the misaligned fibers. Finally, they placed a fluorophore in the medium, and the laser light illuminated the fluorophore across the length of the cell, rather than scattering. The results are detailed in the May 15 issue of PNAS.
“It means that light is being guided from the very beginning to the very end,” Reichenbach said.
However, the researchers discovered more than that during their experiments. For example, they observed that the density of the cones in the retina matches the density of the Müller cells. “This means every cone has its own personal light cable,” Reichenbach said. Because two cones that receive the same part of an entire image with distinct features cannot perceive the different features, the 1:1 ratio of cones to cells explains how cones receive discrete images. The cells also deliver light to rods, and this lowers the threshold for vision under dark conditions, he said.
The scientists noted that Müller cells are not just fiber optic cables, but that together they form a fiber optic plate that transmits images from the input surface of the inner retina to its output surface. Reichenbach commented that it is remarkable that nature invented this device 530 million years before mankind developed it.
They have begun a study of the functional impact of these findings and recently showed that if Müller cells are illuminated by a thin glass fiber, all of their spatially related clonal photoreceptors will be activated, but not those belonging to the neighboring cell. Thus, small and well-defined groups of neurons with their one Müller cell appear to form small, repetitive columnar units that comprise networks in the retina, similar to repetitive units of chips that constitute networks in a computer.
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