ULTRASTRUCTURAL-STUDY OF THE MYOCARDIUM OF THE SINUS VENOSUS AND SINOATRIAL VALVE IN THE DOGFISH (SCYLIORHINUS-CANICULA)

The myocardium of the sinus venosus of the dogfish (Scyliorhinus canic ula) is located between a thick subepicardial collagen-rich layer and a subendocardial network of nerve fibres and ganglion cells. The sinoa trial valve consists of two transversal folds of the cardiac wall whic h are separated by connective tissue; except in their free margins. Th e myocardium of the sinus venosus and the sinusal face of the sinoatri al valve is arranged in bundles which are surrounded by a 40 nm-thick basal lamina. The myocardial cells measure about 7-9 mu m in diameter at the nuclear level. Nerve terminals are frequent in the centre of th e bundles. Most of the sinusal myocardiocytes have a scarce amount of myofibrils which are randomly orientated. The sarcoplasmic reticulum i s relatively well developed and consists of peripheral couplings, subs arcolemmal vesicles, circular and longitudinal tubules. The scarce int ercalated discs show only fasciae adhaerentes. Gap junctions, desmosom es or specific granules are not observed in the sinusal myocardiocytes of the dogfish. In contrast, the atrial myocardio-cytes are smaller, about 5-6 mu m in diameter at the nuclear level. The cytoplasm is dens er and the myofibrils are abundant and orientated in parallel directio ns. Specific granules are present, although scarce. Subsarcolemmal ves icles are less frequent, while the atrial intercalated discs are large r and more abundant than those of the sinus venosus. Neural elements a re scarce in the atrium. The differences observed between sinus venosu s and atrium might be related to the morphological criteria to disting uish between the nodal tissue and the working myocardiocytes of higher vertebrates. On the other hand, we think that the connective tissue p laced between sinus venosus and atrium means that the contraction impu lse generated in the sinus venosus must reach the atrium through the f ree margin of the valve. It might play a role in the sinoatrial valve function.