The Natural Roots of Fiber Optics: You Needn't Go Far to Find Them
Amato, Ivan, Science News
The Natural Roots of Fiber Optics
Optical fibers will one day thread through most household telephone systems, but they won't be the first light-pipes in your home. You're using millions of fiber-optic elements right now to read this sentence. And if your hair is gray, a recent chance observation suggests you may have thousands more optical fibers on top of your head.
Some sicentists describe the retina's 130 million or so light-gathering cells -- called rods and cones -- as biological waveguides, or natural optical fibers. "Photoreceptors act as classical waveguides or fiber-optic elements, [comparable to those] used, for example, by telecommunication systems," write biophysicists Jay M. Enoch and Vasudevan Lakshminarayanan of the University of California, Berkeley, in Vision and Visual Dysfunction (in press, Macmillan, W.N. Charman, Ed.). In 1961, Enoch first reported evidence that vertebrate photoreceptor cells function as natural optical waveguides.
To guide light along a particular axis, a material has to be reasonably transparent. In addition, its index of refraction -- the degree to which light passing through it bends upon exiting into a surrounding medium -- must be greater than that of the surrounding medium. The glass cores of optical fibers for telephonic and other communication technologies, for instance, are denser and more refractive than the glass surrounding them. When light traveling along the core veers slightly off-center and hits the boundary between the two types of glass, it reflects inward rather than passing outward through the boundary. Thus the light remains in the core without leaking out of the fiber's sides.
It now seems rods and cones aren't the only parts of animal anatomy that pipe light around. In a two-paragraph letter published in the March 2 NATURE, John Wells of Berkeley Nuclear Laboratories in Gloucestershire, England, reported discovering that sections of his own hairs--which had begun graying about two years earlier -- could shunt light along their shafts.
That short letter made Enoch wonder if Wells' observation might relate to the fiber-optic behavior of retinal photo-receptors. In an equally brief follow-up letter in the July 20 NATURE, he and Lakshminarayanan suggest an intriguing connection between the two types of natural optical fibers, citing cilia as the link.
Cilia, an 18th-century word for eye-lashes, now refers most often to tiny hair-like structures that nudge substances along through internal organs, help sensory cells gather environmental signals, or "row" tiny creatures through watery environments. Enoch and Lakshminarayanan note in their letter that "many vertebrate sensory systems incorporate cilia in their structure, including the hair cells of the cochlea [in the inner ear] . . . and the photoreceptor cells of the retina."
In an embryo, they point out, the photoreceptor-lined retina develops from surface (ectoderm) cells, some of which also form the subsurface skin layer from which body and head hair sprout. "The infolding and incorporation of surface tissue within the embryo accounts for the presence of ciliated cells within nervous tissue," they write.
Unlike retinal photoreceptors, which use modified cilia to gather and process light, hair might seem an unlikely candidate for an optical fiber. Its more obvious roles, such as conserving body heat, overshadow such seemingly farfetched functions as piping light around. Indeed, Wells wasn't expecting any revelations when he first plucked a hair from his crown, snipped it about a quarter-inch from the root, mounted the segment through a piece of cardboard and put the assembly under a microscope.
"Just out of interest, I wondered what would happen if I shined light into the [hair] specimen directly," Wells told SCIENCE NEWS. To be sure, this experiment didn't come totally out of the blue. For several years, Wells has been looking into ways of using plucked hairs as a biological radiation dosimeter. …