Fate of the Sinus Venosus (Formation of the Right Atrium)

Unlike the atria, the sinus vinosus remains a paired structure with right and left horns. Each horn receives venous blood from three vessels:

• Vitelline vein
 

• Umbilical vein
 

• Commom cardinal vein

Communication between the sinus venosus and the primitive atrium, the sinoatrial oriface, is centrally located.

Gradually the sinoatrial oriface shifts to the right, due to the shunting of blood to the right, until the sinus venosus communicates with only the right atrium. The fate of each structure is as follows:

• the right sinus horn becomes enlarged.
 

• the right anterior cardinal vein becomes the superior vena cava.
 

 • the right vitelline vein becomes the inferior vena cava.
 

• the right umbilical vein is obliterated.

Conversely, the left vein counterparts are obliterated and the left sinus horn diminishes in size and forms the coronary sinus and the oblique vein of the left ventricle.Internally, the sinoatrial oriface is flanked by two valves, the right and left venous valves.Superiorly these two valves meet to form the septum spurium. Note that the left horn opens up underneath the oriface of the right horn (sinoatrial oriface). This is the orifice of the coronary sinus.

Further into development the right sinus horn is incorporated into the expanding right atrium. As the atrium expands the smooth tissue of the sinus venosus displaces the trabeculated tissue of the primitive right atrium anteriorly and laterally where it becomes the adult right auricle. The smooth tissue forms part of the atrium called the sinus venarum. Crista Terminalis, a ridge of tissue located to the right of the sinoatrial oriface, forms the boundry between the auricle and the sinus venarum.

Pulmonary Veins (Formation of the Left Atrium)
Development of the left atrium occurs concurrently with that of the right atrium. During the early part of the fourth week an outgrowth of the pulmonary veins appear from the left atrium. This “sprout” will bifurcate until there are four veins. These vessels will then grow towards the lung buds.

Formation of the left atrium

The left atrium begins to expand gradually intussuscepting the four branches. As the atrial wall expands, the smooth tissue of the pulmonary veins is incorporated into the wall of the atrium and displaces the trabeculated tissue anteriorly and laterally which will then form the adult auricles.Compare this process to the formation of the adult right auricle.

Atrioventricular Canals

Recall that the proximal bulbus cordis gives rise to the right ventricle. Thus, blood flows from the primitive atrium to the left ventricle then to the right ventricle. There is no direct communication between the atria and the right ventricle even after the formation of the bulboventriclular loop.

The atrioventricular canal must shift to the right in order to achieve communication to the right ventricle in addition to the left ventricle. During this shift the proximal bulbus widens and the bulboventricular flange begins to recede. Swellings of mesenchymal tissue, the endocardial cushions, appear on the borders of the atrioventricular canal. There are four cushions: inferior and superior (ventral and dorsal), left and right. The first appear before the latter. These swellings give the atrioventricular canal a “dog’s bone” shape.At approximately day 42 the superior and inferior cushions fuse forming a right and a left atrioventricular canal. The left atrium communicates with the left ventricle and the right atrium communicates with the right ventricle. The shifting process brings the conus cordis to lie superior to the interventricular foramen, which at this point, has not yet been obliterated. The fused endocardial cushions are also responsible for the closure of the ostium primum by fusing with the free edge of the septum primum.