Ultrastructural study of the sinus venosus in embryos of the dogfish (Scyliorhinus canicula).

An ultrastructural study of the development of the sinus venosus has been carried out on seven embryos of the dogfish (Scyliorhinus canicula L.) between 10.5 and 69 mm of total length (TL). The sinus venosus appears at the end of the looping process of the cardiac tube, namely in the 10.5 mm embryo, when the heart reaches its adult tetracameral S-form. The endocardium of the smallest embryo is constituted of a single layer of cubic cells. In larger embryo, these cells progressively acquire a squamous appearance as well as electron-dense cytoplasmic inclusions. The subendocardium is progressively populated by ganglion cells, Schwann cells and bundles of amyelinic fibers that can first be recognised in the embryo of 34 mm TL. Some subendocardial mesenchymal cells located in earlier embryos close to the entrance of the ducts of Cuvier might be their ectomesenchymal progenitors. The myocardium is initially constituted of a single layer of cubic cells. In the embryos of 19, 27 and 34 mm TL, the myocardium becomes multilayered, and the myocardiocytes develop myofibrils randomly arranged throughout the sarcoplasm. In later embryos, the myocardiocytes are innervated and arranged in oval bundles surrounded by a basal lamina. The epicardium covers the sinus venosus by the retrograde migration of the epithelium already established around the atrioventricular groove and, in a lesser degree, by the adhesion of mesothelial cells that are floating free in the pericardial cavity. This process has finished in the embryo of 34 mm TL. The differentiation of the sinus venosus (including the endocardial and myocardial differentiation as well as the epicardial covering) progresses in an anteroventral-posterodorsal direction.