Scientists at Tufts University have grafted eyes onto the rear ends of some experimental tadpoles, according to a paper published online in the journal Nature . What they discovered from the way the tadpoles adapted to this rather odd arrangement may have implications for human organ transplants in the future.
The Tufts team started their experiments by removing the eye tissue from the heads of several embryonic tadpoles and then grafting pieces of it back onto other parts of their bodies, including their posteriors. Those tadpoles whose bodies accepted the grafts eventually grew full-sized amphibian eyes in the transplant locations, some of which even had a rudimentary ability to see.
As a rule, growing grafts in embryonic tissue is easy enough; the hard part is getting it connected to the animal’s central nervous system from the new location. Only a small number of the tadpoles who were given nothing but the graft surgery have managed to grow the necessary neural connections to actually use their rear-facing eyes.
The same problem bedevils other types of grafts, including human organ transplants. In many cases, the reason an implanted organ or tissue graft fails to thrive comes down to its inability to connect with the nervous system and get the signal input healthy tissue needs to survive. Along with rejection, this can be a major reason for disappointing outcomes from such surgeries.
What’s different about these tadpoles is that among those treated with a migraine medication called zolmitriptan, between 29 and 57 percent later acted as though their butt-mounted eyes were fully functional, if a little inconveniently placed. To test the tadpoles’ eyesight, researchers devised a battery of tests such as training the tadpoles to prefer blue lights to red and encouraging them to swim in certain directions when shown spinning patterns.
The study’s authors believe that the zolmitriptan, which works by stimulating the production of the neurotransmitter serotonin, may have triggered the tadpoles to grow extra nerve cells to connect the eyes to the spine, and then it may have stimulated the brain to treat the resulting input as normal eyesight, regardless of the direction the eyes were facing.
If this is indeed what’s going on, these results may shed light on a novel approach to replacing damaged livers, kidneys and other organs in humans. If it ever becomes possible to kick start the human nervous system to forge new connections with transplanted organs and then to treat those tissues like a normal, healthy part of the host’s own body, rejection-resistant transplants may draw measurably closer. Though, if they approach from behind, it may be the helpful little tadpoles of Tufts who see it first.