Septum Formation In The Common Atrium

At the end of the fourth week, a sickle-shaped crest grows from the roof of the common atrium into the lumen. This crest is the first portion of the septum primum (Figs. 11.12 A and 11.14, A and B). The two limbs of this septum extend toward the endocardial cushions in the atrioventricular canal. The opening between the lower rim of the septum primum and the endocardial cushions is the ostium primum (Fig. 11.14, A and B). With further development, extensions of the superior and inferior endocardial cushions grow along the edge of the septum primum, closing the ostium primum (Fig. 11.14, C and D). Before closure is complete, however, cell death produces perforations in the upper portion of the septum primum. Coalescence of these perforations forms the ostium secundum, ensuring free blood flow from the right to the left primitive atrium (Fig. 11.14, B and D).

When the lumen of the right atrium expands as a result of incorporation of the sinus horn, a new crescent-shaped fold appears. This new fold, the septum secundum (Fig. 11.14, C and D), never forms a complete partition in the atrial cavity (Fig. 11.14G). Its anterior limb extends downward to the septum in the atrioventricular canal. When the left venous valve and the septum spurium fuse with the right side of the septum secundum, the free concave edge of the septum secundum begins to overlap the ostium secundum (Fig. 11.12, A and B). The opening left by the septum secundum is called the oval foramen (foramen ovale). When the upper part of the septum primum gradually disappears, the remaining part becomes the valve of the oval foramen. The passage between the two atrial cavities consists of an obliquely elongated cleft (Fig. 11.14, E-G) through which blood from the right atrium flows to the left side (arrows in Figs. 11.12B and 11.14E).

After birth, when lung circulation begins and pressure in the left atrium increases, the valve of the oval foramen is pressed against the septum secundum,

Ostium primum

A Endocardial cushion

Septum secundum

Septum primum

Endocardial cushion

Venous valves

Septum primum

Septum primum

A Endocardial cushion

Septum primum

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