A study was made of the ultrastructural and histochemical characteristics of atrial muscle cells. The myofibrils of these cells do not converge at the nuclear poles as in the ventricular cells, but leave large sarcoplasmic spaces in the central cores, which contain mitochondria, small amounts of rough-surfaced sarcoplasmic reticulum, free ribosomes and one or more well developed Golgi complexes. Numerous cytoplasmic granules, many of which are closely associated with the Golgi material, are present in these cells. These granules can be demonstrated in paraffin sections by the Bowie stain. The smooth-surfaced sarcoplasmic reticulum of atrial fibers consists of a meshwork of interconnected tubules which pass uninterruptedly from one sarcomere to another. No transverse dilatations or T tubules are present as in ventricular cells; however, there are numerous subsarcolemmal cisterns consisting of flattened dilatations of sarcoplasmic reticulum which lie in close proximity to the internal surface of the sarcolemma. There is considerable variation from one cell to another in the number and compactness of arrangement of the myofibrils, and in the abundance of other cellular components.
On the basis of the above findings, we suggest that atrial muscle cells may have a secretory as well as a contractile function.