Chapter 118. Development of Sinus Venosus and Associated Veins - Review of Medical Embryology Book

The sinus venosus contributes to the definitive form of the atrium. It maintains its paired condition longer than any other part of the heart tube, and in the 4 mm embryo consists of a small transverse part, opening into the center of the primitive atrium, and the right and left sinus horns. Each horn receives blood from 3 major veins: the vitelline (omphalomesenteric), umbilical, and common cardinal veins

THE COMMUNICATION BETWEEN THE SINUS VENOSUS AND ATRIUM, which at first is wide, becomes narrow and shifts to the right as a result of development of a deep fold, the sinoatrial fold, which separates the left part of the sinus from the left side of the atrium. In addition, the right horn enlarges due to 2 left-to-right shunts of blood, and by the end of week 4, the right horn is much larger than the left. Thus, the sinoatrial opening shifts to the right and opens into the future right atrium

The first left-to-right shunt of blood results from transformation of the vitelline and umbilical veins

The vitelline veins enter the embryo with the yolk stalk, pass through the septum transversum, and enter the sinus venosus. Between the yolk sac and the septum, the paired vitelline veins are connected via anastomoses. In the septum, the veins are broken up into sinusoids by the developing cords of liver cells, which later become the hepatic sinusoids
The terminal part of the inferior vena cava forms from the right vitelline vein between the liver and right horn of the sinus venosus. The hepatic veins form from the remains of the right vitelline vein in the area of the developing liver. The portal vein forms from the anastomotic network formed around the duodenum by the vitelline veins
The umbilical veins

The right umbilical vein and the part of the left between the liver and the sinus venosus degenerate
The persistent part of the left umbilical vein carries all the blood from the placenta to the fetus
The ductus venosus forms in the liver and connects the left umbilical vein with the inferior vena cav The ductus serves as a bypass through the liver, enabling blood to bypass the liver and go from the placenta to the heart

The second left-to-right shunt of blood occurs when the anterior cardinal veins become connected by an oblique anastomosis which shunts blood from the left to right anterior cardinal vein. The anastomosis becomes the left brachiocephalic vein

The right anterior cardinal and right common cardinal veins become the superior vena cava
The right posterior cardinal vein forms the root of the azygos vein
The posterior cardinal vein contributes to the formation of the left superior intercostal vein
The left anterior cardinal vein vanishes
The left common cardinal vein is greatly reduced at week 10 to form the oblique vein of the left atrium

DUE TO THE SHUNTS AND OBLITERATION of the left umbilical vein at the 5 mm stage and the left vitelline vein at the 7 mm stage, the left horn of the sinus decreases greatly in size and loses its importance. The right horn enlarges to receive all the blood from the head and neck via the superior vena cava and from the lower body region and the placenta via the inferior vena cava. When the left common cardinal vein is finally obliterated at week 10, the distal part of the left sinus horn remains as the oblique vein of Marshall, while the proximal part of the horn and transverse part of the sinus become the coronary sinus

Due to obliteration of the veins on the left side, the right sinus horn and veins greatly enlarg Subsequently, the right horn, the only connection between the original sinus venosus and atrium, is slowly incorporated into the right atrium