Embryonic Lens Prompts Eye Development

Rcjohnsen rcjohnsen at aol.com
Wed Aug 2 01:33:07 EST 2000


DEVELOPMENTAL BIOLOGY:
Embryonic Lens Prompts Eye Development

Elizabeth Pennisi
Sci 289:531 7-28-00
    A blind cave fish is providing new insight into how eyes come to be. In
work reported on page 631, developmental biologists Yoshiyuki Yamamoto and
William Jeffery of the University of Maryland, College Park, show that the lens
plays a leading role in eye development in this fish. If it doesn't form
properly, the researchers found, the embryo will not go on to make the cornea
and other eye structures.
    During the 1960s, work in Russia and Spain had suggested such a 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."
    The Maryland team has been studying the fish, which is called Astyanax
mexicanus, for the past 6 years. Several dozen isolated populations of the
species 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 large eyes.
    To begin to understand this difference, Jeffery and Yamamoto first
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 found, 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.
    Because eye development seemed to progress normally until the lens
degenerated, 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. "It's not a complicated experiment,
but it really [was] very elucidative," says Mathers.
In both types of transplants, the lens 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. These results show that "the lens
plays a central role" in determining whether the eye develops, comments David
Beebe, a developmental biologist at Washington University School of Medicine in
St. Louis. Jeffery doesn't know, however, whether the fish can actually see, as
a vision test is quite difficult to devise.
    Other recent work by Jeffery and his colleagues may explain why the lens
undergoes programmed cell death in the cave fish. The researchers looked at
early embryos for changes in the expression of a variety of proteins that help
specify how cells differentiate into specific organs and tissues. As they
reported last month in Boulder, Colorado, at the annual meeting of the Society
for Developmental Biology, cave fish embryos seem to make more of a protein
called Sonic hedgehog in the area destined to be the head. As a result, fewer
cells are set aside to form the eyes (Science, 23 June, p. 2119). Jeffery
suspects that with fewer cells to start with, the precursor lens may wind up
smaller than usual, perhaps too small to survive, and therefore decays. "It's
possible you are looking at a single gene defect that has caused a drastic
developmental change," Mathers notes.
    Still unclear, however, is how the embryonic lens of the sighted surface
fish triggers further eye development. Presumably the lens produces a molecular
signal, which Jeffery and his colleagues hope to identify eventually. They also
hope to pinpoint the genes involved in eye development in A. mexicanus.
Studying different populations of the fish may provide clues to these genes,
notes Beebe: Because populations became isolated when the fish could see and
became blind independently, different mutations may be involved in each
population.

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Related articles in Science:

Central Role for the Lens in Cave Fish Eye Degeneration.

Yoshiyuki Yamamoto and William R. Jeffery 
Science 2000 289: 631-633. (in Reports) [Abstract] [Full Text] 
Volume 289, Number 5479, Issue of 28 Jul 2000, pp. 522-523. 
Copyright © 2000 by The American Association for the Advancement of Science.  



















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