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An Eye in Your Tail

March 29, 2013

Can you imagine having an eye somewhere else in your body– and using it to see? Researchers claim to have made a tadpole do just that. They used a tadpole of the frog Xenopus laevis, a common model system for vertebrate development. They used blind frogs, and inserted a bit of embryonic eye tissue in the tail. The tissue developed into an “ectopic eye” (see eye in the tail above, at right).

In some tadpoles, the eye developed nerves that reached the spinal cord. Other eyes did not. But the tadpoles with the spinal-connected eyes could distinguish red light from blue light, in a stimulus-reward test. That implies that the tadpoles could “see” with their tails, even though the ectopic eyes do not directly connect to the brain, the way eyes normally do.

Think of the possibilities this suggests for science fiction.

  1. Alex Tolley permalink
    March 29, 2013 1:56 pm

    I think the part that amazes me is the growth of nerves to the spinal cord, “wiring up” the eye, however crudely. This self organization is extremely interesting and suggests very robust self-assembly of biological parts.

  2. March 29, 2013 8:07 pm

    All this says is that when a nerve signal is sent, the brain notices. I’d be very surprised if the tadpole experienced “vision”. Much more likely it interpreted the signal as its tail itching or in pain or something. Why is this more significant than attaching an electrode to the tadpole’s tail and zapping it to create learning?

    I venture to say that if you grafted an eyeball on a human spinal column, the nerve signals it sent would be interpreted as pain.

    • Alex Tolley permalink
      March 29, 2013 9:55 pm

      The scientist report noted that question. At this point all we are sure of is that the ectopic eyes that made connections to the spinal cord were able to generate signals that discriminated between red and blue light, that the tadpoles could discriminate. It might well just be that the signal is experienced as touch, rather than some other sensory modality.

  3. March 29, 2013 8:11 pm

    And pain in an entirely unrelated body area, too!

  4. March 29, 2013 9:53 pm

    What was remarkable was that the tadpole could tell the difference between red light and blue light. That suggests that tadpole received a signal with visual relevance.

    • March 29, 2013 10:01 pm

      John is correct that we do not know that. The eye is equipped to distinguish color and the so the nerves that are fired would carry the signal. But how teh tadpole interpreted that signal is not known. The author suggests that color is somehow perceived, but conservatively, we can only assume that the signals were distinguishable by the tadpole. For example, a blindfolded human swimmer could learn to alter course if given different touch sequences to the skin.

  5. March 29, 2013 10:32 pm

    Extending on this, IIRC each visual receptor has its own neuron. If my hypothetical eye was grafted onto my spine, it certainly wouldn’t be remarkable if the signals were spliced into two different areas of the spinal cord, so that when it saw blue light my right big toe hurt, and when it saw red light, my left thigh itched. I’m pretty sure I could use that as a learning experience, and I certainly wouldn’t call it “vision”. Note also that my skin nerves are sensitive to infrared, but nobody would say that I was “seeing” with my skin, even though I could reliably turn to face the sun while blindfolded.

  6. March 29, 2013 10:57 pm

    Well, it becomes a semantic question then. There are invertebrates that “see” all over with rudimentary eyes on their skin. There are rattlesnakes that “see” infrared through pits in their skin. The vertebrate eye develops, in part, as an in-fold of the skin.

    What’s amazing is that the transplanted eye can rewire itself into the spinal cord–something we thought it never evolved to do.

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