Fig. 4

Further evidence of cell cycle re-entry in C2C12 myotube cultures. a Schematic diagram of plasmid vectors used to label cells. With these vectors, myoblasts express GFP. Upon differentiation, cells express MCK, leading to the expression of Cre and the excision of the GFP gene. Differentiated myotubes and myocytes are thus labeled with RFP. b BrdU was added to the cultures at 72 h and cells were fixed at 96 h and stained for BrdU incorporation (green). Arrow and arrowhead identify the same myocytes in all four panels at 96 h, showing nuclear BrdU expression in the RFP-expressing cells. At the same time, the bottom panels also show cytoplasmic diffuse GFP expression in neighboring myoblasts