Electron Microscope Studies on the Cardiac Conduction System of the Dog

Electron microscope observations were made on the specialized tissues from the distal portion of the common bundle, the bundle branches, the false tendons, and their subendocardial ramifications in the dog heart. All these tissues have shown certain common structural features ; some of them being obviously different from the ultrastructure of ordinary cardiac muscle cells. The specialized musculature is also cellular in nature ; each cell being joined end to end at the intercalated discs and separated from the interstitium by the sarcolemma. The intercalated discs are identical in structure with those of ordinary myocardial cells. The sarcoplasm is occasionally large and the sarcolemma appears to be relieved of its typical " scalloped " outline except where the contracted myofibrils approach the sarcolemma. Although the myofibrils have the same structure as in ordinary myocardial fibers and run in general along the long axis of the cells, they are occasionally scanty and may branch in various directions in the sarcoplasm. The sarcosomes appear to be significantly smaller in size than in ordinary cardiac muscle cells. The smooth surfaced elements of the sarcoplasmic reticulum seem to be arranged in the surroundings of myofibrils as in ordinary myocardial cells and are occasionally conspicuously developed in the enlarged sarcoplasm and very close to the sarcosomes and pinocytotic vesicles. One of the characteristic features of the Purkinje fibers of this animal is the presence of rough surfaced elements of the sarcoplasmic reticulum. Some of them tend to be in parallel arrangement. With prior lead hydroxide staining, dense granules, 12-90m/μ in diameter, may occur in the sarcoplasmic matrix. Although their occurrence varies from section to section, they may represent deposits of glycogen. Occasionally a Golgi apparatus is seen in the perinuclear sarcoplasm. The structure of nuclei in the Purkinje fibers is similar to that of ordianry myocardial nuclei. Besides capillaries, fibrocytes and collagen fibrils, non-myelinated nerve fibers are occasionally found in the vicinity of the sarcolemma.