It is the transparent part of the eye, but for scientists its origin was anything but clear.

Now researchers have pinpointed how the cornea, the thin covering on the outside of the eye that lets in light, manages to be completely see-through.

The discovery could lead to potential cures for eye disease and possibly even cancer.

Unlike almost every other part of the body, the cornea has no blood vessels, and therefore no color. While that much was known, scientists couldn't figure out how the body kept blood vessels from growing there.

The new research shows the area harbors large stores of a protein that binds to growth factors, material the body produces to stimulate blood-vessel formation.

The protein forms a sort of lock on the growth factors, so no blood vessels are produced, leaving the area totally colorless.

Inside the Eye

"Drugs designed to manipulate the levels of this protein could heal corneas that have undergone severe trauma or help shrink tumors fed by rapidly growing abnormal blood vessels," said Reza Dana, head of the Cornea Service at the Massachusetts Eye and Ear Infirmary and an associate professor at Harvard Medical School. "In fact, the next step in our work is exactly this."

The new discovery, which Dana and colleagues called unexpected, will be published this month in the Proceedings of the National Academy of Sciences.

The discovery could lead to research aimed at finding a way to restrict blood vessel growth using the body's own mechanisms.

A breakthrough on that front could in turn be valuable in fighting tumors, which rely on a steady blood supply and can cause blood vessels to grow uncontrollably.

Now that scientists have identified an off switch for blood-vessel production, the next step, they say, is to direct it at places in the eye or elsewhere where blood vessels are growing and the body would simply be better off without them.

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