Evidence for a structural relationship between sarcoplasmic reticulum and Z lines in dog papillary muscle

Electron micrographs of dog papillary muscle fibers show the sarco‐plasmic reticulum (SR) as an extensive three‐dimensional network of diverging and converging tubules that is continuous throughout each sarcomere and in successive sarcomeres. The network surrounds each myofibril and is continuous across the fiber. Tubules of the SR network are oriented transversely and longitudinally. Interconnections of transversely and longitudinally oriented SR tubules form circular pathways in the network. A structural relationship appears to exist between the Z line and longitudinally and transversely oriented tubules of the SR network at their points of apposition with the Z line. A rather constant space approximately 100 Å wide is seen between SR tubules apposed at the Z line in cross‐sections and in favorable longitudinal sections of fibrils. This space is traversed by electron‐opaque strands which appear to be connections between SR tubules and the Z line. Other lines of evidence support the view that Z lines and SR tubules are connected. One of these is the change of position of Z line SR tubules in relation to the change of position of Z lines in adjacent fibrils. Another line of evidence is the continuity of SR apposed at the Z line and SR connected to the sarcolemma.