Tadpoles Able To See With Eyes Transplanted Onto Their Back
February 28, 2013

Franken-Froggies See With Eyes Transplanted Onto Their Backs

Brett Smith for redOrbit.com - Your Universe Online

In a study that could have implications for organ transplant procedures, two scientists from Tufts University found that eyes placed on the backs of tadpoles were still capable of vision despite having no direct neural connection to the brain.

"A primary goal in medicine is to one day be able to restore the function of damaged or missing sensory structures through the use of biological or artificial replacement components,” said Michael Levin, director of the Center for Regenerative and Developmental Biology at Tufts University. “There are many implications of this study, but the primary one from a medical standpoint is that we may not need to make specific connections to the brain when treating sensory disorders such as blindness."

According to the team´s report in the Journal of Experimental Biology, they began their investigation by surgically removing the eyes of embryonic tadpoles and grafted them onto the posterior of recipient embryos. The procedure induced the growth of ectopic — or abnormally placed — eyes, which were neurologically connected directly to the spinal cord rather than the brain. The natural eyes of the recipient tadpoles were then removed, leaving only the ectopic eyes.

Using fluorescence microscopy, the biologists found various patterns of nervous activity around the ectopic eyes, but none of the activity showed a connection to the brain or cranial region.

To test whether the posterior-placed ectopic eyes could pick up visual information, the team first placed the tadpoles into individual Petri dishes that were illuminated with either red or blue light. They found that the red light caused the tadpoles to be calm, swimming about the dish slowly. Under blue light, however, they became slightly agitated.

Next, the researchers wanted to see if the tadpoles could use their ectopic eyes to pick up visual cues. They developed a computerized system designed to condition the tadpoles by administering a mild electric shock when they swam too close to a red light.

The team found that“¯almost 20 percent of the animals showed learned responses and began to avoid the red light. This behavior was similar to that of a control group of tadpoles with naturally developed eyes. Eyeless tadpoles or tadpoles that were not conditioned using electrical shocks did not exhibit the response.

"This has never been shown before," Levin said. "No one would have guessed that eyes on the flank of a tadpole could see, especially when wired only to the spinal cord and not the brain."

"One of the big challenges is to understand how the brain and body adapt to large changes in organization," said Levin´s co-researcher Douglas J. Blackiston. "Here, our research reveals the brain's remarkable ability, or plasticity, to process visual data coming from misplaced eyes, even when they are located far from the head."

"Ectopic eyes performed visual function," Blackiston added. "The brain recognized visual data from eyes that impinged on the spinal cord. We still need to determine if this plasticity in vertebrate brains extends to different ectopic organs or organs appropriate in different species."

The researchers suggest that future studies should investigate how the brain recognizes visual signals coming from tissue near the gut.