Doctors in Shanghai, China, are working out a treatment plan for a three-armed baby called Jie-jie. The 2-month-old child has a normal right arm opposite a pair of somewhat normal-looking left arms. He doesn't have full use of either left arm. Do extra limbs ever work?
If they do, it's very rare. Jie-jie's extra limb doesn't seem to work, but his doctors still say they have "no record of any child with such a complete third arm." For an extra arm to function properly, it would need to develop bones and muscles and connect up to the nervous system. It also has to make joints at the elbow and wrist. Once the arm comes together, it may not have a shoulder to which it can attach. That's because shoulders and arms come from different groups of embryonic cells. A birth defect that duplicates an arm won't necessarily make an extra shoulder.
Doctors are far more accustomed to seeing extra fingers—or "polydactyly"—than extra arms. That's because it's easier to make a finger than an arm. Fingers form out of a paddle of flesh at the end of each developing arm. A group of cells at one side of the paddle releases a chemical called "sonic hedgehog"—yes, it's named after the video game—that spreads to the other side in diminishing concentrations. The chemical gradient tells some cells to die off, while others remain alive. This has the effect of sculpting individual fingers—or the spaces between them—from tissue that's already in the right place. An extra finger forms when something goes wrong with this signaling pathway. You might get a second thumb, for example, if cells on the wrong side of the paddle start producing sonic hedgehog.
Extra fingers don't work too well, either. In some cases, the second thumb or pinky will be little more than a nub of flesh without bones or muscles, or it might not have its joints. In some types of polydactyly, the "extra" thumb will work, but it won't be able to move without the regular thumb moving at the same time.
The complexity of limb development makes third arms very unusual. But scientists can easily induce them in lab animals by grafting tissue from one part of an embryo to another. They can also spark the formation of an inappropriate "limb bud" by applying chemical signals to embryonic tissue. Frogs sometimes develop extra limbs in the wild, as a result of parasitic worm infections. Tadpoles pick up worm larvae, which form cysts that split the developing frog legs in two.
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Explainer thanks Tony Scialli of Sciences International and Clifford Tabin of Harvard Medical School.