Abstract
Although biological cells are mostly transparent, they are phase objects that differ in shape and refractive index. Any image that is projected through layers of randomly oriented cells will normally be distorted by refraction, reflection, and scattering. Counterintuitively, the retina of the vertebrate eye is inverted with respect to its optical function and light must pass through several tissue layers before reaching the light-detecting photoreceptor cells. Here we report on the specific optical properties of glial cells present in the retina, which might contribute to optimize this apparently unfavorable situation. We investigated intact retinal tissue and individual Müller cells, which are radial glial cells spanning the entire retinal thickness. Müller cells have an extended funnel shape, a higher refractive index than their surrounding tissue, and are oriented along the direction of light propagation. Transmission and reflection confocal microscopy of retinal tissue in vitro and in vivo showed that these cells provide a low-scattering passage for light from the retinal surface to the photoreceptor cells. Using a modified dual-beam laser trap we could also demonstrate that individual Müller cells act as optical fibers. Furthermore, their parallel array in the retina is reminiscent of fiberoptic plates used for low-distortion image transfer. Thus, Müller cells seem to mediate the image transfer through the vertebrate retina with minimal distortion and low loss. This finding elucidates a fundamental feature of the inverted retina as an optical system and ascribes a new function to glial cells.
Highlights
B iological cells and tissues are usually fairly transparent due to the lack of strong intrinsic chromophores in the visible part of the spectrum and especially in the near-infrared
What these examples have in common is a relatively regular geometry of the light-guiding structures and, in the case of living cells, a sophisticated specialization for this very function. Considering these facts, it seems surprising that the retina in the vertebrate eye is inverted and that images projected onto the retina have to pass several layers of randomly oriented and irregularly shaped cells with intrinsic scatterers before they reach the light-detecting photoreceptor cells [7, 8]
As a first step to characterize the retina as a phase object, we investigated freshly dissected guinea pig eyes by using modified transmission microscopy (Fig. 1 a and b)
Summary
B iological cells and tissues are usually fairly transparent due to the lack of strong intrinsic chromophores in the visible part of the spectrum and especially in the near-infrared. Other natural optical fibers occur in deep-sea glass sponges or in the compound eye of insects, whose biomimetic copies have even found their way into technical components [5, 6] What these examples have in common is a relatively regular geometry of the light-guiding structures and, in the case of living cells, a sophisticated specialization for this very function. Considering these facts, it seems surprising that the retina in the vertebrate eye is inverted and that images projected onto the retina have to pass several layers of randomly oriented and irregularly shaped cells with intrinsic scatterers before they reach the light-detecting photoreceptor cells [7, 8]. It is intriguing to investigate whether they could play a role in the transfer of light through the inner retina
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