The myocardium of the sinus venosus of the dogfish (Scyliorhinus canicula) is located between a thick subepicardial collagen-rich layer and a subendocardial network of nerve fibres and ganglion cells. The sinoatrial valve consists of two transversal folds of the cardiac wall which are separated by connective tissue, except in their free margins.
The myocardium of the sinus venosus and the sinusal face of the sinoatrial valve is arranged in bundles which are surrounded by a 40 nm-thick basal lamina. The myocardial cells measure about 7-9 μm in diameter at the nuclear level. Nerve terminals are frequent in the centre of the bundles. Most of the sinusal myocardiocytes have a scarce amount of myofibrils which are randomly orientated. The sarcoplasmic reticulum is relatively well developed and consists of peripheral couplings, subsarcolemmal vesicles, circular and longitudinal tubules. The scarce intercalated discs show only fasciae adhaerentes. Gap junctions, desmosomes or specific granules are not observed in the sinusal myocardiocytes of the dogfish. In contrast, the atrial myocardiocytes are smaller, about 5-6 μm in diameter at the nuclear level. The cytoplasm is denser and the myofibrils are abundant and orientated in parallel directions. Specific granules are present. although scarce. Subsarcolemmal vesicles are less frequent, while the atrial intercalated discs are larger and more abundant than those of the sinus venosus. Neural elements are scarce in the atrium.
The differences observed between sinus venosus and atrium might be related to the morphological criteria to distinguish between the nodal tissue and the working myocardiocytes of higher vertebrates. On the other hand, we think that the connective tissue placed between sinus venom and atrium means that the contraction impulse generated in the sinus venosus must reach the atrium through the free margin of the valve. It might play a role in the sinoatrial valve function.