Seeing with one eye ... outside the head

Jason Bittel / Slate /

Published Mar 6, 2013 at 04:00AM

PITTSBURGH — Recently, we have witnessed remarkable, fictional-sounding advancements in science and medicine. For our next trick, though, we’re going to need a bucket of tadpoles with eyes on their butts and some good old-fashioned alternating current.

Using embryos from the African clawed frog (Xenopus), scientists at Tufts’ Center for Regenerative and Developmental Biology were able to transplant eye primordia — basically, the little nubs of flesh that will eventually grow into an eye — from one tadpole’s head to another’s posterior, flank or tail. They don’t play around with nerve endings or “wiring” or anything like that. They just cut out the cells from the head, slice open a bit of the tail and jam them in.

As the eyes grow, they send out snaking tendrils of nerve fiber, or axons. We know this because the “tissue donor” tadpoles were injected with tdTomato, a fluorescent red protein. This allowed the researchers to watch innervation, or nerve growth, as it happened.

Before they could test the ectopic eyes, however, the native ones had to be severed and removed. Otherwise, how would the scientists know which of the tadpole’s three eyes was truly seeing? (Note: The tadpoles received anesthesia via fish sedative, and wounds healed completely within 24 hours.)

Finally, it was time to put the cyclopses to the test. Using an underwater arena rigged with blue and red LEDs and electric shock, scientists ran through an exhaustive array of controls and variables.

Interestingly, the tadpoles with no eyes at all could still react to LED changes, revealing that they may have other ways of sensing light. However, they proved woefully inadequate at avoiding being shocked, showing that whatever information they were getting was ultimately flawed or unusable. On the other end of the spectrum were the control tadpoles that quickly learned to avoid the shocks through the scientists’ regimen of aversive conditioning.

As for the main event? Amazingly, a statistically significant portion of the transplanted one-eyes could not only detect LED changes, but they showed learning behavior when confronted with electric shock.