Developmental aspects of the sinus valves and the sinus venosus septum of the right atrium in human embryos

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Abstract
In 32 human embryos from 5 to 27 mm of length, stages 13 to 23 (according to the Carnegie system of stages), the contributions of the sinus venosus septum and the right sinus valve of the right atrium to the formation of the Eustachian and Thebesian valve were examined by scanning electron microscopy. The sinus septum takes part in the subdivision of the right sinus valve into the Eustachian and the Thebesian valves. From its first origin the sinus septum forms a septal structure between the orifices of the right hepatic vein (hepatic portion of the inferior caval vein), the precursor of the inferior caval vein, and the left horn of the sinus venosus, the precursor of the coronary sinus. Before the incorporation of the sinus venosus into the right atrium, it has an intra-sinusal position, and extends between the bases of the left and the right sinus valve. During the incorporation of the sinus venosus into the right atrium the sinus septum receives an intra-atrial position, and its positional relationships to the sinus valves and the orifices of the corresponding veins remain unchanged in principle. Due to the connection between the sinus septum and the right sinus valve, after completion of the incorporation of the sinus, the superior portion of the right sinus valve branches y-like into a lateral limb, (i.e. its original inferior portion) and into a medial limb, (i.e. the sinus septum). Thus, the Eustachian valve is formed by the superior portion of the right sinus valve and the sinus septum; whereas the Thebesian valve is formed by the inferior portion of the right sinus valve. The normal variations in the configuration of the Eustachian and the Thebesian valves are supposed to be caused by differential growth of the constituents of the different portions of the valves.