Effects of serum in culture medium on muscle fiber dedifferentiation. (A) Representative confocal maximum intensity projection images immunostained for α-actinin (green), myosin (red), and nuclei (blue) in muscle fibers cultured for 7 days in serum-free medium and medium containing 10% FBS. Note the lack of sarcomere misalignment in the muscle fiber cultured in serum-free medium and the large dedifferentiated mass in the muscle fiber cultured in 10% FBS. (B–E) Contractile measurements of muscle fibers cultured in low serum, medium containing 10% FBS, and serum-free medium, measured at days 1 and 7 of culture. (B) Quantification of resting sarcomere length at days 1 and 7 ex vivo culture. (C) Quantification of percentage of sarcomere shortening at days 1 and 7 ex vivo culture. (D) Quantification of contractile velocity at days 1 and 7 ex vivo culture. (E) Sarcomere length during maximal contraction of muscle fibers kept in culture. Data are means ± SEM; large dots represent mean values per mouse, and small dots represent single muscle fibers. Significance was determined using a two-way ANOVA with P < 0.05 considered as significant with * = P < 0.05, ** = P < 0.01, and *** = P < 0.001.