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Tadpole Sees Through Eyeball on Its Tail

A rose by any other name would smell as sweet—but does an eye in any other place see as sharply? The answer may be yes—a finding that could help people with blindness or other eye disorders, a new study says. Recent experiments show that tadpoles bred with eyes surgically implanted in their tails instead of...

A rose by any other name would smell as sweet—but does an eye in any other place see as sharply? The answer may be yes—a finding that could help people with blindness or other eye disorders, a new study says.

Recent experiments show that tadpoles bred with eyes surgically implanted in their tails instead of their heads still see—the first discovery of its kind.

A tadpole with an eyeball growing on its tail. Image courtesy D. Blackiston and M. Levin

 

Remarkably, despite their placement far from the brain, these out-of-place eyeballs—or “ectopic eyes”—still worked by sending signals to the brain via the spinal cord. (See Sea Urchin Body Is One Big Eye.”)

“Not all the tadpoles with the ectopic eyes could see,” study leader Michael Levin, director of the Tufts Center for Regenerative and Developmental Biology, said in an email.

“But of the ones that could, their performance was very similar to those with normal eyes.”

More Than Meets the Eye

Because of their structural similarities with human eyes, frog eyes are often used for modeling eye disorders.

For the experiment, the team surgically grafted “eye primordia” from donor tadpole embryos onto host embryos, 95 percent of which grew eyes on their tails.

The team then used red and blue LEDs to condition the tadpoles to associate red light with a mild electric shock.

Consequently, both tadpoles with normal eyes and those with ectopic eyes developed an aversion to red light and learned to avoid red-lit areas—meaning that the tadpoles with tail “eyes” could see. In contrast, a control group of eyeless tadpoles did not learn to avoid red light.

The spine is known to transmit all kinds of sensory information throughout the body, but the 64,000-eyeball question is how the brain recognizes the signals from the far-flung ectopic eyes for what they are. (Also read: “Eyes Made of Rock Really Can See, Study Says.”)

“What is really interesting is how the brain knows it is visual data,” said Levin, whose study appeared February 27 in the Journal of Experimental Biology.

“My hypothesis, although we have no proof of it yet, is that these—and probably many other organs’—signals are tagged with ‘metadata’—there is something about these signals that reveals the type of organ they come from.” (Watch a video of a tadpole developing.)

Eye to the Future

The study has far-reaching implications for the treatment of human blindness and beyond, the authors say.

“In the short term, it might mean that ectopic organs could be connected to [a person’s] spinal cord,” said Levin, so the person wouldn’t “need brain surgery to make [the organs] functional.” (Read “See-Through Vision Invented.”)

The research could even expand to include “cybernetic hybrid devices” like electronic eyes or hands, he said.

“Not only is this relevant for biomedical treatment of injuries and birth defects where the normal organs are damaged [or] missing, but [it’s also relevant] for sensory augmentation with new technologies,” said Levin.

For instance, recent breakthroughs have allowed quadraplegic patients to control robotic limbs with their thoughts alone.

Overall, the future for altering and repairing our bodies in new ways looks bright, he said.

“Beyond all this, understanding whether and how the brain builds a map and communicates with body organs will really revolutionize our understanding,” he said, “as well as impact our ability to induce bodies to regenerate missing organs themselves.”

It’s exciting, futuristic stuff—and may even finally provide hope for those who’d like eyes in the back of their heads.

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Meet the Author

Stefan Sirucek
Stefan Sirucek is a writer and journalist who reports from both sides of the Atlantic. He's written for the Huffington Post and Wall Street Journal. Follow him on Twitter at @sirstefan.