Pax7 directs postnatal renewal and propagation of myogenic satellite cells but not their specification.

The paired-box transcription factor Pax7 has been claimed to specify the muscle stem cell lineage since inactivation of Pax7 led to a failure to detect muscle satellite cells. Here we show that muscles of juvenile Pax7(-/-) mice at P11 contain a reduced but substantial number of satellite cells. Neither juvenile nor adult Pax7(-/-) mice displayed a significant reduction in the number and size of myotubes, indicating that the remaining number of satellite cells sufficed to allow normal postnatal muscle growth. The number of satellite cells in Pax7 mutant mice declined strongly during postnatal development, although single satellite cells were readily identified in adult Pax7 mutant mice. Muscle regeneration was impaired in adult Pax7 mutant mice. Our results clearly indicate an essential function of Pax7 for renewal and maintenance of muscle stem cells and exclude an exclusive role of Pax7 in satellite cell specification.