Blind Tadpoles See with Eyes in the Back of Their Spines

Some amphibians give a whole new meaning to having eyes in the back of their heads. Researchers have transplanted eyeballs to the spines of blind tadpoles, allowing them some degree of vision. This is the first ever evidence that functional sight can occur so far from the brain, and the findings may give rise to improved regenerative therapies in humans.

In total, scientists experimented with 134 African clawed frog tadpoles. They carefully grafted new eyes onto places such as their torsos and tails, and then surgically removed their original eyes. The new eyes came from tadpole donors that were genetically modified to generate a red fluorescent protein, so that researchers could tell under a microscope whether these eyes sent red nerves outward in the body.

The researchers then tested these tadpoles for vision. The weird-eyed tadpoles were placed in a circular arena that was half illuminated with red light and half with blue light. Software regularly switched what color light the different areas received. When tadpoles entered the red light area, they received a small electrical zap. The researchers also employed a motion-tracking camera to keep track of where the tadpoles were.

While most of the tadpoles couldn't actually see, some surprisingly could. The scientists found that six tadpoles that had eyes implanted in their tails could apparently see since they chose to remain in the areas with blue light. In addition, researchers noted that half of the recipient tadpoles had no red nerves grow from their eyes, while a quarter had nerves projecting toward the gut and another quarter had nerves extending to their spine.

"These findings suggest that the brain has remarkable plasticity and may actually take a survey of its body configuration to make use of different body arrangements," said Michael Levin, one of the researchers, in an interview with LiveScience. "If it were not the case, then every time a mutation produced an improvement in body plan-a large, significant change in anatomy-the animal would die and the beneficial mutation would be lost."

The findings could have large implications for regenerative medicine, and could allow scientists to eventually replace damaged sensory and motor organs. In addition, it has implications for augmentation technology-improving senses in humans.

The findings are published in the Journal of Experimental Biology.