Cut the tail off a Xenopus tadpole and it quickly makes a new muscle-packed tail. But are the muscles in this regenerated appendage formed from de-differentiated myofibres (as in newts) or from stem cells? On p. , Jonathan Slack and colleagues provide strong evidence that muscle satellite cells - adult stem cells that repair damaged muscles in mammals - rebuild the muscles in regenerating Xenopus tails and reveal the role that the Pax7 transcription factor plays in this process. The researchers demonstrate that regenerating Xenopus tails contain many dividing muscle satellite cells, most of which express Pax7. Using Pax7 as a marker for satellite cells,they show that these cells are responsible for forming the muscle masses of the regenerated tail. Finally, they report that when pax7 function is antagonized during tail regeneration, the tail reforms but it contains fewer satellite cells. Thus, Pax7 is needed to maintain satellite cells as a stem cell population but is not required for their differentiation into myofibres.
