The black blob on the tadpole's tail is its new eye.
(Credit: D Blackiston and M Levin)
Scientists have been able to successfully create a new eye on a tadpole, through which it appears to be able to see in spite of the absence of an optic nerve.
If asked whether you could see through eye cells someone had transplanted into your knee or finger, we would have scoffed at the very thought. Apparently, though, the idea is not as far-fetched as we might have assumed, as demonstrated by biologists at the Tufts University Center for Regenerative and Developmental Biology.
Taking the embryos of African Xenopus frogs, the team — led by Douglas J Blackiston, PhD — removed optic primordia (that is, the tissue that will form the eyes) and simply grafted them to the subject's posterior, tail or sides.
These cells developed into ectopic eyes — none of which connected to the subject's brain, though the optic nerves did connect to the spine.
In order to make sure that the experiment's results were as accurate as possible, the subjects' natural eyes were then removed.
In order to test whether these eyes worked, the team developed a visual training system. Quadrants of the tank were lit by red or blue LEDs; tadpoles swimming in the red segments were administered a mild electric shock. Their movement was monitored by a motion-tracking system.
Four groups were tested: normal tadpoles (the control group); tadpoles with implanted ectopic eyes; tadpoles with no eyes at all; and normal tadpoles that were not shocked.
The results were exciting: over 19 per cent of the tadpoles with ectopic eyes swam away from the red zone in exactly the same manner as the control group, learning that red meant pain, while blue was fine. This behaviour was not demonstrated by the other two groups — indicating that the ectopic-eyed tadpoles had some method of detecting light not available to the completely eyeless ones.
Blackiston said, "Ectopic eyes performed visual function. The brain recognised 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."
What the scientists don't know is how the brain recognised, in the absence of an optic neural connection, how exactly the brain recognised the signal received as visual. There's a lot more research to be done.
Meanwhile, the results of this study can be found in The Journal of Scientific Biology, titled "Ectopic eyes outside the head in Xenopus tadpoles provide sensory data for light-mediated learning".
We look forward to receiving our very own butt eyes sometime in the distant future.