Figure 1

MD6.0-lacZ reporter is specifically associated with fibre nuclei. A, Cross-section through the TA muscle of an MD6.0-lacZ adult transgenic mouse reacted with X-gal. Note the abundant and occasionally central (arrow) βgal⁺ nuclei indicating that some reactive nuclei are within fibres. B, Single muscle fibre extracted from the EDL of an adult MD6.0-lacZ mouse, cultured for three days in growth medium and reacted with X-gal. The fibre (arrow) contains βgal⁺ nuclei, whereas non-differentiated satellite cells coming from this fibre do not contain βgal (arrowheads). Insets: proliferating satellite cells on a cultured fibre express MyoD but not MD6.0-lacZ. MyoD in fibres is obscured by X-gal stain. C-F, Cell cultures from P0 MD6.0-lacZ limb muscle homogenates, grown for six days in growth medium and differentiated for one day (C, E) or 10 days (D, F). Cultures were reacted with X-gal (blue, C-F) and stained for desmin (red) to label myogenic cells, and for all MyHC (green) to label differentiated cells (E, F). Myonuclei weakly βgal⁺ (concave arrowhead, stain visible only in E) or strongly βgal⁺ (arrows) are in differentiated cells, whereas undifferentiated myogenic cells are βgal^- (arrowhead). After ten days differentiation, the majority of differentiated cells were strongly βgal⁺. G, MyHC expression precedes βgal accumulation as cultures differentiate. Desmin⁺/MyHC^- cells (black bars) lack βgal. Mononucleated and multinucleated MyHC⁺ myocytes (white and grey bars, respectively) accumulate βgal with time in differentiation medium. Values above columns are the total numbers of cells observed in several dishes from three separate experiments. H, P0 MD6.0-lacZ culture after two days differentiation showing βgal (blue) only in a multinucleate cell, but MyoD (brown) in both myoblasts (concave arrowheads) and myotubes (arrow). Fibroblasts are unstained (arrowhead). Note that myotube nuclei unlabeled for βgal may be newly fused.