Get ready for custom eyeball transplants for people who absolutely must have eyes in the backs of their heads — or pretty much anywhere on their bodies. Researchers at Tufts University just published a paper where they report transplanting working eyes onto the tail of a blind tadpole. Here's how they did it.
Xenopus tadpoles arise from a genus of frogs native to Sub-Saharan Africa. Within this genus of aquatic frogs is Xenopus laevis, better known as the African clawed frog. To carry out the study, researchers at Tufts removed the eyes from several Xenopus tadpoles used in testing. Then they transplanted a primordial eye harvested from an embryonic tadpole onto the tail of the newly-blind tadpoles. Positioning the eye on the tail was important, because it contains the nerve-heavy spine.
Using a set of blinded tadpoles with eyes attached to their tails, the researchers color-coded a tank to determine if the "tail eyes" would function.
The authors of this study previously published work on the use of aversion techniques to train tadpoles, which laid the foundation for this eye-to-tail transplantation study. Using very mild electric shocks, the researchers trained the tadpoles to avoid red-colored regions, with 19% of tadpoles that underwent surgery successfully trained. Neither of the control groups — eyeless tadpoles and tadpoles that did not receive an electric shock — displayed any aversion to color-coded areas. In other words, the tail-eye tadpoles were clearly seeing the red color.
But here's what's really weird. Examination of fluorescent proteins within the transplanted eyes revealed that no nerve connections between the tail-eye and tadpole's brain formed after transplantation. The eyes functioned even though they weren't connected to the amphibians' brains. These tadpoles could see without using their brains.
Could similar transplants be performed on vertebrates and higher level organisms in the future? Possibly. Proving that sensory input doesn't require a direct connection to the brain could make transplants a lot easier than previously believed. Michael Levin, co-author of the study, notes a possible extension to humans, saying:
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. 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.
This is an excellent step forward in transplantation. Now we have evidence that sensory organs can be grafted onto some animals without a direct connection to the brain — and they still work.
Plus, non-facial eyeballs could become a new plastic surgery trend. You know you want to have those eyeball hands like the monster in Pan's Labyrinth.
Top image (also repeated in the body) courtesy of D. Blackiston and M. Levin at Tufts University. Additional image from Michael Linnenbach/CC. If you would like to read more about this very interesting research endeavor, check out the Journal of Experimental Biology for Blackiston and Levin's article, Ectopic eyes out side the head in Xenopus tadpoles provide sensory data for light-mediated learning.