Transmission electron microscopy (TEM) analysis for mitochondria morphology and content and expression of muscle fiber type-specific genes in TWEAK-KO mice. Soleus muscle of six-month-old wild-type and TWEAK-KO mice were isolated and longitudinal sections were processed for TEM analysis. (A) Representative images of longitudinal soleus muscle are shown. The abundance and the size of mitochondria were increased in TWEAK-KO mice compared to wild-type mice. Arrows point to mitochondria in muscle sections. Circles are used to show representative enlarged mitochondria. Stars (‘*’) point to subsarcolemmal space in longitudinal sections. Scale bar: 1 μm. (B) Quantification of the number of subsarcolemmal mitochondria in soleus muscle of wild-type and TWEAK-KO mice. (C) Quantification of the number of intermyofibrillar mitochondria in soleus muscle sections of wild-type and TWEAK-KO mice. (D) Relative mRNA levels of MyHC type I, IIA and IIB in gastrocnemius muscle of 4.5-month-old wild-type and TWEAK-KO mice. Data are represented as mean ± SD. N = 3 in each group. *P <0.05; values vary significantly from wild-type mice. KO, knockout; TWEAK, TNF-like weak inducer of apoptosis.