The introduction to this article provides insufficient context for those unfamiliar with the subject. Please help improve the article with a good introductory style. (October 2009)

Septum primum
Interior of dorsal half of heart from a human embryo of about thirty days. (Septum primum labeled at center top.)
Gray's	subject #135 512
Carnegie stage	13
Days	29
Gives rise to	atrial septum

In the developing heart, the cavity of the primitive atrium becomes subdivided into right and left chambers by a septum, the septum primum, which grows downward into the cavity. The increasingly smaller gap below it (before it fuses with the endocardial cushion) is known as the ostium primum (ie. "the first opening"). The septum primum eventually fuses with the endocardial cushion, closing the ostium primum off completely. Meanwhile, perforations appear in the superior part of the septum primum, forming the ostium secundum (ie. "the second opening"). This will eventually form part of the fossa ovalis.

[edit] External links

• Overview at um.edu.mt
• Diagram at lww.com
• MedEd at Loyola GrossAnatomy/thorax0/Heart_Development/Atria.html - "Atrial Partitioning"

This article was originally based on an entry from a public domain edition of Gray's Anatomy. As such, some of the information contained within it may be outdated.

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