Qualitative evaluation of T-tubule/SR junctions (i.e., CEUs) within the I band of solues muscle fibers. (A–H) Representative EM images in longitudinal sections of soleus fibers from WT (A and D), CASQ1-null (B, E, and F), and dCASQ-null (C, G, and H) mice at 4 mo of age in which T-tubules are stained in black with ferrocyanide. White arrows point to longitudinal triadic T-tubules, whereas white asterisks point to transversal T-tubule extensions at the I band. (D–H) SR membrane forming flat cisternae at the I band are false labeled in yellow. White numbers mark each SR cisternae element. Scale bars: A–C, 0.5 μm; D–H, 0.1 μm.