Embryological Development
During development, the atria initially share a common chamber. As the two begin to separate, the septum is not fully formed to allow blood shunting from the right to left atrium in order for blood to bypass the lungs. The septum primum and spina vestibuli grow together to divide the right and left atria during the fifth week of development. This results in the formation of the foramen primum. The septum primum will continue to grow until the sixth week when the dorsal and ventral cushions will fuse to form the atrioventricular septum. The fusion process closes the foramen primum via controlled cell death (apoptosis) and several smaller perforations come together to form the foramen secundum. The perforations form before the foramen primum closes completely, ensuring the shunting process continues at all stages.
Read more about this topic: Foramen Secundum
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