A blind cave fish has provided new insight into how eyes come to be. In the 28 July issue of Science , developmental biologists show that the lens plays a leading role. If the lens doesn't form properly, the researchers found, the fish embryo will not make the cornea and other eye structures.
The cave fish, Astyanax mexicanus, is a useful creature for studying what it takes to make an eye. Several dozen isolated populations exist in northeastern Mexico, with some living in surface ponds and streams and others in caves and underground waterways. Over the past million years or so, the eyes of the underground fish have degenerated to varying degrees, while the surface fish have retained their vision.
To begin to understand the nuts and bolts behind this difference, Yoshiyuki Yamamoto and William Jeffery of the University of Maryland, College Park, monitored eye development in the blind fish. They observed a precursor lens and the rudiments of the optic cup forming during the embryo's first 24 hours. But soon afterward, they noticed, the cells in the embryonic lens underwent programmed cell death. Other eye structures, such as the cornea and the iris, never appeared, and the retina never developed distinct, organized layers, as it does in normal eyes. The eyeball gradually sank back into the socket and was covered by a flap of skin.
Jeffery and Yamamoto wondered whether this disintegration was triggered by a signal from the embryo or from the lens itself. To find out, Yamamoto removed the embryonic lens from one eye of a blind cave fish embryo and replaced it with a lens from a surface fish embryo. He also did the opposite experiment, replacing the lens of an embryonic surface fish with one from a cave fish embryo. In all cases, he labeled the transplanted tissue with dye so he could track what happened to it.
The lens always behaved as if it were still in its original embryo. The one from the cave fish degenerated, even though it was in an environment conducive to further development, whereas the lens from the surface fish thrived in the cave fish embryo and the eye differentiated, forming a cornea, anterior chamber, and iris. Jeffery doesn't know, however, whether the fish can actually see, as a vision test is quite difficult to devise.
During the 1960s, work in Russia and Spain had suggested a central role for the lens, but this new study "nails it," says Peter Mathers, a developmental biologist at West Virginia University School of Medicine in Morgantown. What's more, he adds, because the eye develops similarly in all vertebrates, including humans, "the implications are much broader than [for] just the cave fish."
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