The structure of smooth and striated portions of the adductor muscle of the valves in a scallop

The smooth and cross-striated portions in the scallop's adductor muscle of the valves are examined by thin sectioning and freeze-fracture. Three types of junctions are identified on the surface membrane: (1) peripheral couplings between elements of the sarcoplasmic reticulum and plasmalemma—these are more numerous in the tonic smooth than in the phasic cross-striated adductor; (2) fasciae and hemifasciae adherentes, which allow in series and in parallel adhesions from cell to cell and from cell to connective tissue, respectively—the structure is similar in both types of fibers, but hemifasciae are more numerous in the smooth portion; (3) gap junctions—these are quite numerous in both types of fibers. Their presence is correlated to the fact that end plates are rare, and fibers are shorter than the length of the muscle. In the cross-striated muscle evidence is presented for the intercalated growth of sarcomeres.