(A–J) H&E-stained TA muscle sections from WT (A, C, E, G, and I) and p94KI (B, D, F, H, and J) mice of the ages indicated. p94KI showed typical dystrophic phenotypes, such as cell invasion (dotted circles), central nuclei (black arrowheads), and the splitting of myofibers (white arrowheads). Scale bars: 100 μm. (K and L) Increased CNM during aging in p94KI mice. The number of CNM in the TA (K) and soleus (L) was compared with the number of total myofibers. (M) Frequency distribution of myofiber CSArs in the soleus of mice over 90 weeks old. A total of 302 fiber profiles was traced in each section from 3 mice of each type. The average myofiber CSAr was 1,542 μm2 in WT mice and 1,342 μm2 in p94KI mice. The fiber size in p94KI mice was significantly smaller than in WT mice (P < 0.01). White bars, WT; black bars, p94KI. (N) The mean serum CK level (IU/l) in p94KI mice appeared higher than in WT, but this difference was not statistically significant. *P < 0.05 versus WT. (O and P) Ultrastructural analysis showed a typical striated pattern with properly assembled sarcomeres in a 91-week-old p94KI mouse (P) with no apparent difference from the pattern of a 26-week-old WT mouse (O). Scale bars: 1 μm.