COLLEGE PARK, Md. - The old cliche "the blind leads the blind" may no longer apply to a population of cave fish. Eye parts lost during the past million years of evolution were restored in just a matter of days after a lens transplant from a sighted surface-dwelling fish of the same species (Astyanax mexicanus), according to a University of Maryland biology research report featured in the July 28 issue of Science.
This eye-opening result sheds light on understanding the role of genetic factors in eye growth and development according to William Jeffery, Maryland professor and biology chair, who coauthored the Science paper with Yoshiyuki Yamamoto, a postdoctoral researcher in Maryland's biology department.
Within eight days of implanting a lens from a sighted surface-dwelling fish to a blind cave fish of the same species, Maryland researchers began to see an eye develop from underneath a flap of skin. After two months, the cave fish had grown a large restored eye with a distinct pupil, cornea and iris. In addition, the retina of the restored eye showed rod photoreceptor cells, which are rare in the degenerate cave fish eye.
"This offers clues about what sort of molecules are involved in eye growth of any vertebrate and it shows the growth of an eye is controlled in a large part by the lens," said Jeffery.
Although Jeffery and Yamamoto can't say whether cave fish regain sight after having a restored eye, this research suggests a simple method in testing factors that control eye-growth.
"Our current research focuses on identifying basic development mechanisms in embryos that can be studied in the laboratory. Though we are not working with human patients, these findings could someday prove useful to our colleagues in clinical practice," said Jeffery.
Thousands of fish gathered in Mexican caves
During this research, Jeffery and his students collected thousands of cave fish from seven different caves in northeastern Mexico. These ghostly, pale fish live only in dark caves, depend on an acute sense of smell to find food and are not a target for predators, which are rarely present in caves.
"Our system deals with fish of the exact same species, but their living space and behavior greatly differ. What makes this kind of research especially interesting is determining what makes them different," said Yamamoto.
The Science paper notes cave fish begin to form eyes as embryos, but the young lens deteriorates and the cornea, iris, pupil and other optic tissues remain undeveloped. In adult cave fish, the degenerate eye sinks into the orbit and is covered by a flap of skin.
When the scientists reversed the experiment, where surface-dwelling fish received the regenerated lens from a cave fish, the transplant failed to trigger any significant eye growth.
According to the researchers, these results show cave fish have lost the ability to promote eye development. With these findings, the scientists conclude, "evolutionary changes in an inductive signal from the lens are involved in cave fish eye degeneration."
However, Jeffery and Yamamoto caution the possibility of other factors contributing to eye loss is currently under investigation in their laboratory. The researchers are hopeful if they can stop cave fish lens from triggering eye regression, they can learn exactly how the mechanism works. Research described in the Science paper was supported by the National Science Foundation.
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