Rush Center for Congenital
and Structural Heart Disease




The atria and atrial septum

The atria of the mature heart are derived from more than one origin.  The trabeculated portions (appendages) of the right and left atria are from the primitive atria, while the smooth walled posterior portions of the left and right atria originate from incorporation of venous blood vessels.  The posterior aspect of the left atrium is formed by incorporation of the pulmonary veins, while the posterior smooth portion of the right atrium is derived from the sinus venosus. 

The two sinus horns continue to be a paired structure after fusion of the two endocardial tubes, later they fuse to give a transverse sinus venosus.  The entrance of the sinus venosus shifts rightwards to eventually enter into the right side of the common atrium exclusively and the veins draining into the left sinus venosus (left common cardinal, umbilical and vitelline veins) degenerate and the left sinus venosus will become smaller as it will only drains the venous circulation of the heart, becoming the coronary sinus. 

The sinus venosus orifice into the right atrium is slit like and to the right of the yet to develop septum primum [362].  The sinus venosus now connecting to the right atrium will assume a more vertical position and the sino-atrial junction will become guarded by two valve like structures resulting from the invagination of atrial wall at the right and left sino-atrial junction. This orifice will eventually enlarge and the superior and inferior vena cava and the coronary sinus open separately and directly into the right atrium.  The right and left sinoatrial valves join at the top forming the septum spurium, this septum and the two sinoatrial valve like structure obliterate and are not appreciated in the mature heart [366].

Atrial septation starts when the common atria becomes indented externally by the bulbus cordis and truncus arteriosus, this indentation will correspond internally with a thin sickle shaped membrane, called septum primum, developing in the common atrium at day 28 of development [375, 378].  The septum primum divides the atrium into right and left chambers.  It grows from the postero-superior wall and extends towards the endocardial cushion of the atrioventricular canal.   The septum primum initially has a concave shaped edge growing towards the atrioventricular canal; the orifice connecting the two atria is called the ostium primum.  As the superior and inferior endocardial cushions fuse at about day 42 of development [375], the atrioventricular canal id divided into right and left orifices, the concave lower edge of the septum primum fuse with the endocardial cushion, resulting in obliteration of the ostium primum.  However, just before this happens fenestrations appear in the postero-superior part of the septum primum forming.  The fenestrations coalesce and form one orifice, called the ostium secundum.  The ostium secundum  maintains a communication between the two atria [366].  The ostium secundum and superior vena cava later acquire a more anterosuperior position, although they maintain their relationship to each other.  This is achieved through growth of the atria [366].

As the fenestrations within the septum primum coalesce, another sickle shaped membrane develops on the anterosuperior wall of the right atrium at about 42-43 days of development.  Septum secundum is just right of the septum primum and left to the sinus venosus valve.  As the septum secundum grows, it covers, but does not obliterate the ostium secundum, therefore, allowing continuation of blood passage from the right to left atria.  The septum secundum grows towards the endocardial cushion until leaving only an area at the posterosuperior part of the interatrial septum where the septum primum continue to exist as the foramen ovale membrane.  The septum primum disappears from posterosuperior portion of interatrial septation and the edge of the septum secundum forms the rim of the fossa ovalis [370]. This is completed around day 43 of development (Figure 9).



Figure 9

The atrial septum forms from two sources: the septum primum and septum secundum.  (a) Initially the atrium is a common chamber.  (b) Evidence of septation starts around day 35 of development as a sickle shaped septum primum grows towards the fusing endocardial cushion.  (c)  As the septum primum fuses with the endocardial cushion, fenestrations form to maintain a communication between the two atria.  (d)  The fenestrations coalesce, followed by growth of a septum secundum to the right of the septum primum.  (e) Septum secundum maintains an orifice creating a tunnel like communication with ostium secundum, known as foramen ovale.