St. Louis, Jan. 27, 1997 -- Researchers at Washington University School of Medicine in St. Louis have solved a centuries-old puzzle: Do both our eyes develop from a single precursor or does each eye develop from a separate structure?
"This question arose long ago because some infants are born with a single, Cyclops-like eye," says Yi Rao, Ph.D., assistant professor of anatomy and neurobiology. "Our work shows that the embryo has a single eye field that normally separates into two. If this fails to happen, cyclopia occurs."
The findings are published in the Feb. 1 issue of Development. Rao's graduate student, Hua-shun Li, is lead author. Jane Y. Wu, Ph.D., assistant professor of pediatrics and molecular biology & pharmacology, is a collaborator.
Most cyclopic humans die before or shortly after birth, but their existence in reality as well as myth sparked a debate about whether two normal eyes fuse or a single eye fails to split into two. "It was not until the beginning of this century that embryologists began to subject this problem to experimental tests," Rao says.
Rao realized he could explore eye development after he discovered a gene he named ET, which is expressed early in embryonic development. The gene produces a protein belonging to a new family of transcription factors called T domain proteins, which bind to other genes and turn them on. "Most likely ET controls eye formation," Rao says.
This molecular marker made it possible to locate the part of the embryo that develops eyes. By tracking the gene's product, the researchers were able to see the eye field of frogs change from a band into two spots over the course of a few hours.
The researchers obtained the same result when they repeated the study with another gene, Pax-6, which is known to regulate eye development in vertebrates and invertebrates.
They also asked why two eye spots form. "We found that an inhibitory signal shuts off ET expression in the middle of the eye field," Rao says.
The signal came from the prechordal mesoderm, which lies beneath the center of the eye field. When the researchers removed this tissue, the eye field formed, but it did not divide into two, so the resulting tadpole was cyclopic. Similar experiments showed that the same mechanism operates in chick embryos.
Two recent discoveries suggest the mesodermal signal could be made by a gene called sonic hedgehog, which is expressed in prechordal mesoderm and is known to regulate nervous system development. In 1996, collaborators in Bethesda and Baltimore found that mice without this gene develop cyclopia. The same year, researchers in Toronto, St. Louis and Philadelphia identified a mutation in the sonic hedgehog gene as the cause of a birth defect called holoprosencephaly, in which the forebrain fails to cleave into two hemispheres. This condition occurs in 1 in 250 miscarried fetuses and 1 in 16,000 live births. Cyclopia accompanies severe holoprosencephaly.
The work with humans and mice and the current study with frogs and chicks suggest that the interaction between the eye field and sonic hedgehog may be a general phenomenon. "We think it may apply to all vertebrates," says Rao.
Images of the frog embryo are available.
Li H, Tierney C, Wen L, Wu JY, Rao Y. A single morphogenetic field gives rise to two retina primordia under the influence of the prechordal plate. Development. 1997;124(3); 603-615.