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Seeing the light

Photonics Spectra
Nov 2008
Patricia A. Vincent, patricia.vincent@laurin.com

Scientists have long assumed that, because C. elegans lives in the soil and spends most of its life in the dark, the 1-mm eyeless roundworm has no light-detecting cells. But a research team from the University of Michigan in Ann Arbor, led by Shawn Xu and including Jie Liu and Alex Ward, reasoned that there must be a mechanism that acts to keep the worms in the dark.

The group directed a light beam at the heads and tails of the worms under a microscope. When light touched the head of a forward-moving worm, the worm reversed its course and wriggled away; similarly, when the light touched the tail of an animal that was moving backward, that animal began moving forward. The strongest response was to ultraviolet-A radiation, which is fatal to C. elegans if exposure is prolonged.

C.-Elegans_head_illus.jpg


The researchers surmised that this light sensitivity – called negative phototaxis – protects the worms from leaving the safety of their dark environment. When they near the surface, “they see the light and don’t like it,” Xu explained.

To determine which of the worms’ 302 nervous system cells are responsible for this reaction to light, they used a pulsed 435-nm nitrogen laser to destroy the cells one by one and then exposed the worms to UVA radiation. Conversion of light energy into electrical signals – or phototransduction – leads to the light sensitivity and, in studying this process, they discovered that some of the photosensitive receptor cells used by the worms are similar to the cones and rods used by humans.

The researchers suggest that the worms’ primitive visual system resembles the primordial eye described by Charles Darwin in The Origin of Species. The primordial eye contains only two cells: a light-sensitive photoreceptor and a pigment cell that shields it. C. elegans has photoreceptors only, but it is possible that the soil may play the role of the pigment cells, preventing light from striking the photoreceptors unless the worm approaches the surface.

Xu explained that it is a lot easier to study worms than it is humans, and the “knowledge learned from worms may provide novel insights into how human cones and rods function.” Because the chemical reactions employed by C. elegans are similar to those in humans, the worm can be used for research into the building blocks of human vision and for studying how disruption of the phototransduction pathway can lead to eye disease.


GLOSSARY
microscope
An instrument consisting essentially of a tube 160 mm long, with an objective lens at the distant end and an eyepiece at the near end. The objective forms a real aerial image of the object in the focal plane of the eyepiece where it is observed by the eye. The overall magnifying power is equal to the linear magnification of the objective multiplied by the magnifying power of the eyepiece. The eyepiece can be replaced by a film to photograph the primary image, or a positive or negative relay...
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