Figure 6. Pax7 expression marks a differentiation-resistant muscle progenitor cell population.

Human muscle cultures were grown in differentiation medium for 7-10 days to induce myotube formation and then immunostained. (A) Staining with anti-myosin and anti-Pax7 antibodies showed that Pax7 is expressed by some differentiation-resistant cells but never by myotubes. (B) Immunostaining for Pax7 and desmin demonstrated that some cells in the differentiation-resistant population are Pax7⁺/desmin⁺ (arrowhead) while others are Pax7⁺/desmin⁻ (arrow). (C) Cultures were pulsed with BrdU for 24 hours and processed to assess BrdU incorporation and desmin expression. Frequently, mononuclear cells were identified that were BrdU⁺/desmin⁺ (arrowhead) cells or BrdU⁺/desmin⁻ (arrow). (D) Percentage of differentiation-resistant cells expressing Pax7 and BrdU after 10 days in differentiation medium for fetal and adult cultures. (E) Flow cytometry was used to analyze α7 integrin expression among cells in the differentiation-resistant cell population. The data indicates that within the myotube cultures, some cells express a high level of α7 integrin (arrowhead) (presumably differentiated myotubes) whereas a second distinct population of cells retain a low level of α7 integrin expression (arrow) (presumably differentiation-resistant progenitor cells). Scale Bar = 20μM