The three-dimensional fossa ovalis

Sir,

I read with interest the original research titled “Morphological study of fossa ovalis [FO] and its clinical relevance” by Joshi et al in this issue of Indian Heart Journal.¹ This work discusses in depth the anatomy and embryology of fossa ovalis. The clinical perspective unambiguously establishes that fossa ovalis is a three-dimensional structure, though it sounds like a two-dimensional structure.2, 3 This would help the echocardiographer and intervention cardiologist for better understanding of FO. In addition to complementing the author for his unique, first observation about the anatomy of fossa ovalis in Indians, I also want to add one or two key points, which may be interesting to the readers. This comment would feature a little more about the embryology of FO. During the fourth and fifth weeks of embryonic development, coronary sinus and left oblique vein are formed from left horn of sinus venosus. The right horn, which now forms the only communication between the original sinus venosus and the atrium, is incorporated into the right atrium to form the smooth-walled part of the right atrium. Therefore, the right horn of sinus venosus becomes the only gateway for systemic veins into the true single atrium (primitive atrium). Blood shunts from right to left. The sinoatrial orifice is flanked on each side by a valvular fold, the right and left venous valves. Right venous valve forms crista semilunaris, eustachian valve, thebesian valve, and finally merges into tendon of Todaro from rostrocaudal direction. Left venous valve and septum primum form right and left borders of interseptovalvular space. Dorsocranially, the right and left valves fuse, forming a ridge known as the septum spurium.⁴ Initially, the valves are large, but when the right sinus horn is incorporated into the wall of the atrium, the left venous valve and the septum spurium fuse with the septum secundum septum [Fig. 1].

Fig. 1.

Interatrial septum, though it sounds like a two-dimensional structure, it is really a three-dimensional structure. Embryologically, this complex septum is contributed by endocardial cushion (A), septum primum (B), septum secundum (C), and septum spurium (D). Septum spurium is formed by RVV and LVV at their dorsocranial end and left venous valve (E); right venous valve (F) which is the forerunner of crista semilunaris, eustachian valve, and thebesian valve; (G) and (H) mark the left and right borders of interseptovalvular space; (I) the flap valve mechanism in FO, which allows only right to left shunt in embryonic period.

Conflicts of interest

The author has none to declare.