Upper part of septum primum

upper part of septum primum.

•The inter arterial segment develops from the septum primum and septum secundum .

•The septum primum starts to develop from 28 days .
•It is a thin cresentric fold of endocardium that arises craniodorsally and grows down to the AV cushion , leaving a ostium primum below it’s free edge .

•Septum primum fuses with the AV cushion on approximately day 35, obliterating the ostium primum.

•The ostium(foramen) secundum is an opening in the upper part of septum primum. It forms at about day 33 , i.e. just before the closure of ostium primum .

•The septum secundum is a thick muscular septum that arises right to the septum primum.

It grows from the roof of the atria but does not reach the AV cushion.
•The gap between the two septum is known as foramen ovale.

•After birth the foramen ovale gets obliterated by the fusion of septum primum and septum secundum .
known as fossa ovalis

•In adults the annulus/ limbus of fossa ovalis is derived from the lower free margin of septum secundum while the floor of fossa ovalis is derived from the septum primum.