Shape and disposition of clefts, tubules, and sarcoplasmic reticulum in long and short sarcomere fibers of crab and crayfish.

The disposition of surface invaginations (clefts, Z and T tubules) and of the sarcoplasmic reticulum has been examined by electron microscopy at three accelerating voltages (100, 200 and 1000 kV) and by phase-contrast light microscopy in crustacean muscles infiltrated by the "Golgi stain." In long-sarcomere, tonic type fibers, an extensive system of invaginating clefts has been observed, along with both Z and T tubules. Z and T tubules form interconnections with each other, but only T tubules form specific contacts with the sarcoplasmic reticulum, which in these fibers forms an extended and continuously fenestrated network. In short-sarcomere, phasic type fibers, a ladder-like disposition of an abundant T network is found. Z tubules are absent in these fibers. The sarcoplasmic reticulum forms more frequent junctions with flattened areas of T tubules and with clefts, but has less extensive free surfaces than in the long-sarcomere fibers.