Pax7 distribution in human skeletal muscle biopsies and myogenic tissue cultures

Demonstration of the importance of the paired box transcription factor Pax7 for the murine myosatellite cell population, with persistent expression in mature skeletal muscle, prompted us to investigate the distribution of Pax7 protein in biopsy samples of normal and pathological human skeletal limb muscle. Immunostaining for M-cadherin, an adhesion molecule present at the interface between myofibre and satellite cell, and the characteristic position adjacent to the muscle fibre and beneath the fibre's basement membrane were used to identify satellite cells. Anti-Pax7 reactivity was found in the majority of satellite cells but a small population was Pax7 negative. Neither could we identify Pax7-positive nuclei in freshly regenerating myotubes or in presumed myoblasts in these biopsies. Similarly, in myogenic cell cultures derived from the explantation of human foetal muscle Pax7 expression was low or undetectable at the proliferative myoblast stage but it became prominent in an increasing proportion of mononucleate cells after the induction of differentiation. This expression was, however, restricted to mononucleate cells; it did not persist into the differentiation stage of newly formed multinucleate myotubes. Despite this, in the biopsy samples, we occasionally found Pax7-positive nuclei in muscle fibres that seemed to be undergoing degenerative changes. Most of these were found to be the nuclei of cells engaged in focal regenerative processes, but Pax7 re-expression by myonuclei "in distress" cannot be ruled out entirely.