Sarcoplasmic reticulum in the conducting fibers of the dog heart.

The ultrastructure or sarcoplasmic reticulum (SR) was studied in the conducting fibers of the dog heart. A dense network of sarcoplasmic tubules occurred in well preserved cytoplasmic areas. Some of the tubules extended into large sacs filled with finely filamentous material. The sacs often appeared in subsarcolemmal apposition connected to the sarcolemma by electron dense projections. The flattened cisternae of SR occurred beneath the sarcolemma in the myofibrillar region of the cell. Long flattened cisternae only partially apposed to the sarcolemma were quite common in one specimen. Sarcoplasmic reticulum was continuous throughout the interfibrillar spaces surrounding each myofibril. At the level of the Z-line the SR formed a quasi tubular flattened structure surrounding the Z-line and closely adhering to it.