Fig. 4.

In the core of large TAs, SR tubules are regularly arranged, perfectly cylindrical, covered by SERCA particles, and held together by little bridges. a, b Tubules in the center of TAs are arranged in a highly ordered hexagonal pattern (a), giving rise to an electron optical diffraction transform that indexes on 60° angles and shows up to five orders of reflections (b). c The ordered arrangements of tubules is probably maintained, if not established, by frequent cross links of uniform size that bridge the gap between the membrane of adjacent tubules (small arrows). d Tubules in the center of large TAs are perfectly straight resulting—when freeze-fractured and replicated—in regularly alternated views of the luminal and cytoplasmic (large and small arrows, respectively) leaflets. e A diagram modeling the plane of fracture that exposes alternatively luminal and cytoplasmic leaflets of the tubules (large and small arrows, respectively). f The cytoplasmic leaflet is decorated by particles representing small aggregates of SERCA molecules. The particles show tendency towards a helical arrangement (f, arrows), which is best detected by holding the image at eye level and glancing along the direction of the two sets of arrows. g A more precise view of the helical symmetry of SERCA particles (arrows) is visible on the free cytoplasmic surface of an SR tubule isolated from aging muscle and prepared by freeze-drying, rotary shadowing. Barsa, d 0.1 μm; b, e, g 0.05 μm