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 50 discrete Z lines that were examined, only 4 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.
ASJC Scopus subject areas
- Agricultural and Biological Sciences (miscellaneous)