Interaction of the sarcoplasmic reticulum with Z‐lines during myogenesis in amphibian skeletal muscle

Electron microscopic examination of developing myofibrils in regenerating skeletal muscle of Rana pipiens tadpole tails reveals that sarcoplasmic reticulum (SR) is in regular and close association with Z‐lines of young sarcomeres before transverse tubules have formed alongside the myofibrils. SR is also associated with M‐lines of young sarcomeres but on a less regular basis than with Z‐lines. Counts of the number of contact sites of SR with Z‐lines, M‐lines and combined A‐ and I‐bands of longitudinally sectioned sarcomeres of young myofibrils confirms the visual impression that the SR associates with a relatively much greater frequency with Z‐lines and M‐lines than with other regions of the sarcomere. This suggests the formation of a specific bond between SR and Z‐ and M‐lines. Serial transverse sections of developing myofibrils were examined in order to determine whether Z‐lines displayed SR association at all stages in their formation. Of fifty discrete Z‐lines that were examined, only four smaller Z‐lines appeared to lack SR association. An examination of a comparable number of more diffuse Z‐bodies representing an earlier stage of Z‐line formation suggests that no more than one‐half of these Z‐bodies were accompanied by SR. These observations support the conclusion that SR does not initiate formation of Z‐line precursor elements, but rather associates with Z‐lines as they condense from more diffuse Z‐bodies.