Pax7 is required for the specification of myogenic satellite cells

The paired box transcription factor Pax7 was isolated by representational d ifference analysis as a gene specifically expressed in cultured satellite c ell-derived myoblasts. In situ hybridization revealed that Pax7 was also ex pressed in satellite cells residing in adult muscle. Cell culture and elect ron microscopic analysis revealed a complete absence of satellite cells in Pax7(-/-) skeletal muscle. Surprisingly, fluorescence-activated cell sortin g analysis indicated that the proportion of muscle-derived stem cells was u naffected. Importantly, stem cells from Pax7(-/-) muscle displayed almost a 10-fold increase in their ability to form hematopoietic colonies. These re sults demonstrate that satellite cells and muscle-derived stem cells repres ent distinct cell populations. Together these studies suggest that inductio n of Pax7 in muscle-derived stem cells induces satellite cell specification by restricting alternate developmental programs.