Fig. 5.

TAs contain CASQ, but not RyRs. Mitochondria, T-tubules, and triads are only occasionally trapped within the aggregate. a, b Immunostaining of EDL fibers with antibodies against CASQ and RYR reveals that TAs contain CASQ (a, see also f), but not RYR (b). Bright RYR-positive spots (arrow in b), representing trapped triads (see also g), are rare. c, d Two different optical sections of the area marked in b by the dashed box show RYR positive spots at the edges of a optically sectioned TA (c, arrows; see also e) and in a plane grazing the TA (d). e Stacks of SR-T-tubules junctions (empty arrows) containing RYRs are frequently present at the TA’s edge (compare with c). f TAs’ tubules are filled with electron-dense material, most likely CASQ. g–i SR/T-tubule junctions (or triads, g), T-tubules (h), and mitochondria (i) are rarely trapped inside TAs, mostly where smaller TAs merge with each other (see also Fig. 3b). Barsa–d, 5 μm; e 0.5 μm; f–i 0.2 μm