Control of muscle regeneration in the Xenopus tadpole tail by Pax7.

The tail of the Xenopus tadpole will regenerate completely after transection. Much of the mass of the regenerate is composed of skeletal muscle, but there has been some uncertainty about the source of the new myofibres. Here, we show that the growing tail contains many muscle satellite cells. They are active in DNA replication, whereas the myonuclei are not. As in mammals, the satellite cells express pax7. We show that a domain-swapped construct, pax7EnR, can antagonize pax7 function. Transgenic tadpoles were prepared containing pax7EnR driven by a heat-inducible promoter. When induced, this reduces the proportion of satellite cells formed in the regenerate. A second amputation of the resulting tails yielded second regenerates containing notochord and spinal cord but little or no muscle. This shows that inhibition of pax7 action does not prevent differentiation of satellite cells to myofibres, but it does prevent their maintenance as a stem cell population.