Within muscle fibres, tubules appear as microtubules, transverse tubules, and different components of the sarcotubular system, the latter consisting of longitudinal tubules and lateral or terminal sacs which, at special junctional connections, are situated on both sides of transverse tubules forming triads. Pathological structures related to the transverse tubules are honeycomb structures, both the transverse tubules and the honeycomb ones being labelled by extracellularly applied lanthanum [1] or potassium ferrocyanide [2], thus, indicating an open connection between the interior of the muscle fibres and the extracellular space. Dyads or multiple incomplete or complete forms of triads, pentads or heptads, are pathological forms of triads. The pathological equivalent of the sarcotubular system is tubular aggregates. The derivation of tubular aggregates from the sarcotubular system can be established by electron microscopy showing continuity between tubular aggregates and the sarcotubular system where both border, and by immunohistochemical evidence of proteins that are common components of the sarcotubular system and tubular aggregates, i.e. SERCA (sarcoplasmic endoplasmic reticulum calcium ATPase) 1 and sarcalumenin in longitudinal tubules, calsequestrin in the terminal lateral sacs, and ryanodine receptor 1 and the 95 and 31 kDa isoforms of triadin in the junctional parts of the sarcotubular system. Tubular aggregates are highly organised, often in a hexagonal fashion, the individual tubules of which are composed of cylindrically formed membranes. In addition, different components may be encountered within these single-walled tubules, multiple double-walled tubules, filaments, or an amorphous finely granular slightly electron-dense material. Based on these internal differences within the single-walled tubules, they may make up different tubular aggregates or appear mixed within different tubules of the tubular aggregates and have, hence, been differently subclassified as tubules I–III [3] or up to five different forms [4]. While the double-walled tubules