W fibers free of interstitial tissues. Finally, we identified Pax3 cells in the muscle interstitial environment and demonstrated that they coexpressed MyoD during regeneration. Sublaminar satellite cells in hind limb muscle did not express detectable levels of Pax3 protein or messenger RNA. Therefore, we conclude that interstitial Pax3 cells represent a novel myogenic population that is distinct from the sublaminar satellite cell lineage and that Pax7 is essential for the formation of functional myogenic progenitors from sublaminar satellite cells.