The septum primum is a thin, crescent-shaped wall of tissue that grows during early embryonic life to begin dividing the heart’s single atrial chamber into left and right sides. It is the first of two overlapping septa that together form the interatrial septum, and its behavior before birth, during birth, and sometimes long after birth shapes some of the most common congenital heart conditions in humans. Despite being tissue-paper thin, the septum primum plays a surprisingly active role in directing blood flow through the fetal heart, and the way it fuses (or fails to fuse) after a baby’s first breath determines whether the division between the atria holds for life.
How the Septum Primum Forms
Heart development begins remarkably early. In humans, the septum primum starts to appear around the end of the fourth week of gestation. It originates in the dorsocaudal wall of the common atrium, growing downward from the roof of the atrial chamber toward the cushion-like tissue that sits between the developing atria and ventricles. Research on human embryos has shown that the septum primum arises from cells associated with the lower endocardial cushion where it contacts the atrial wall.
Studies in chick embryos have given a detailed picture of where the tissue inside the septum primum comes from. Between roughly day 3 and day 5 of chick development, endocardial cells covering the septum primum become activated: they enlarge, sprout migratory projections, and begin dividing more rapidly. By about day 4 these hypertrophic cells peel away from the endocardial lining and move into the space beneath it, becoming the loose connective tissue (mesenchyme) that fills the growing septum. This process mirrors what happens in the cushion tissue of the atrioventricular canal and outflow tract, suggesting a shared developmental toolkit across several parts of the heart.1PubMed. Origin of mesenchymal tissue in the septum primum: a structural and ultrastructural study
As the septum primum grows toward the endocardial cushions, it leaves a temporary opening at its lower edge called the ostium primum (or primary foramen). This gap allows blood to continue flowing between the two developing atria. Eventually the septum primum reaches the cushions and the ostium primum closes. But just before it does, a series of small perforations appear higher up in the septum primum, coalescing into a new opening called the ostium secundum (or secondary foramen). This sequential open-and-close design keeps interatrial blood flow uninterrupted throughout fetal life.
The Foramen Ovale and Fetal Circulation
Shortly after the ostium secundum opens in the septum primum, a thicker, muscular fold called the septum secundum begins growing from the atrial roof on the right side. The septum secundum never extends all the way down; it leaves a window near its inferior edge known as the foramen ovale. The septum primum drapes across this window on the left-atrial side, acting as a one-way flap valve. During fetal life, when blood pressure in the right atrium exceeds that in the left atrium, oxygen-rich blood coming from the placenta via the umbilical vein and inferior vena cava pushes the septum primum aside and streams from the right atrium into the left. This shortcut is essential because the fetal lungs are not yet doing any gas exchange; the placenta handles that job, and the foramen ovale ensures that oxygenated placental blood reaches the left side of the heart and, from there, the brain and body.
The septum primum is not simply a passive flap pushed around by pressure gradients. Research on human fetal hearts has found that the septum primum contains muscle fibers whose contraction can influence blood flow through the foramen ovale. Its excursion into the left atrium depends partly on left atrial pressure and partly on those intrinsic muscle fibers, making the septum primum an active participant in directing fetal blood flow rather than just a pressure-driven membrane.2Arquivos Brasileiros de Cardiologia. Morphological basis for the study of the interatrial septum in the human fetus
What Happens at Birth
The moment a newborn takes its first breath, the landscape of pressures inside the heart shifts dramatically. The lungs inflate, pulmonary blood vessels open, and blood begins flowing freely through the lungs for the first time. This drops right atrial pressure and raises left atrial pressure. With the pressure relationship reversed, the septum primum is pressed against the septum secundum, functionally closing the foramen ovale. Blood can no longer cross between the atria, and the heart starts operating as two separate pumps for the first time.
Functional closure happens within minutes to hours of birth, but anatomical closure, where the two septa actually fuse together, takes much longer. Studies using three-dimensional reconstructions of neonatal mouse hearts showed a progressive reduction in the size of the interatrial communication over the first week of life, with complete closure of the opening by postnatal day 7. Full fusion of the septum primum and septum secundum occurred between 4 weeks and 3 months of age in those animals.3PubMed Central. Progressive anatomical closure of foramen ovale in normal neonatal mouse hearts In humans the timeline is broadly similar in concept, though it can stretch out: tissue fusion gradually seals the two septa together during infancy and early childhood in most people.
When the Septum Primum Doesn’t Seal
In roughly one in four adults, the septum primum and septum secundum never fully fuse. The result is a patent foramen ovale (PFO), a small tunnel-like passage between the atria that remains probe-patent even though it may be functionally closed under normal pressure conditions. Most people with a PFO have no symptoms and never find out about it. The flap of septum primum stays pressed shut most of the time because left atrial pressure exceeds right atrial pressure in a normally functioning adult heart.
A PFO becomes clinically relevant when something transiently raises right atrial pressure above left, such as coughing, straining, or certain types of exercise. In those moments, blood (and anything it carries, like a small clot) can cross from the right atrium to the left atrium, bypassing the lungs. This is the mechanism behind paradoxical embolism: a blood clot that forms in the veins of the legs or pelvis travels to the right heart, crosses through the PFO, and then reaches the arterial circulation where it can cause a stroke. Transesophageal echocardiography is the preferred tool for evaluating PFO because it allows direct measurement of the maximum separation distance between the septum primum and septum secundum, as well as characterization of features such as tunnel length and whether the surrounding septum is hypermobile.4PubMed. Pearls and Pitfalls in the Transesophageal Echocardiographic Diagnosis of Patent Foramen Ovale
Atrial Septal Defects Involving the Septum Primum
While a PFO is a failure of fusion between two otherwise normally formed septa, atrial septal defects (ASDs) are actual holes in the interatrial wall caused by problems during development. The septum primum figures in more than one type.
The most common variety is the secundum ASD, which sits in the middle of the interatrial septum at the fossa ovalis. It is widely taught that secundum ASDs result from too much resorption of the septum primum during the formation of the ostium secundum, leaving a hole that’s too large for the septum secundum to cover. That explanation accounts for many cases, but not all. An analysis of specimens with secundum ASDs found that in about 86% the defect was traceable to a malformation of the septum primum (the valve of the foramen ovale), while in roughly 14% the problem lay instead with an absent or deficient superior limbus of the septum secundum.5PubMed. Morphogenetic differences of secundum atrial septal defects That distinction matters surgically, because the anatomy around the defect influences which repair strategies work best.
An ostium primum ASD is a different beast. It occurs low in the atrial septum, adjacent to the atrioventricular valves, and reflects a failure of the septum primum to reach and merge with the endocardial cushions during early development. Because the endocardial cushions also give rise to parts of the mitral and tricuspid valves, an ostium primum ASD often comes with abnormalities in one or both of those valves.6PubMed Central. Large Ostium Primum Interatrial Septum Defect in Asymptomatic Elderly Patient This type is part of the broader family of atrioventricular septal defects and is especially associated with Down syndrome.
Genetic and Molecular Drivers of Atrial Septation
The septum primum does not grow and fuse on autopilot. Its development depends on a network of genes and signaling pathways, and when those are disrupted, the septum can fail to form properly. Two transcription factor genes have been especially well studied. Mutations in TBX5 cause Holt-Oram syndrome, which pairs heart defects with limb abnormalities and often includes ASDs. Mutations in NKX2-5 are linked to ASDs accompanied by problems with the heart’s electrical conduction system, such as progressive atrioventricular block.7PubMed Central. Electrical disorders in atrial septal defect: genetics and heritability
Mouse studies have revealed that Tbx5 does some of its critical work not in the developing septum itself, but in a population of progenitor cells called the second heart field. Reducing Tbx5 activity specifically in second heart field cells caused ASDs in mice, even when Tbx5 levels were normal in the heart muscle and endocardium. The affected progenitor cells showed problems progressing through the cell cycle. Tbx5 appears to work alongside hedgehog signaling in these progenitors: artificially boosting hedgehog pathway activity rescued the septal defects caused by Tbx5 loss.8PubMed Central. Tbx5-hedgehog molecular networks are essential in the second heart field for atrial septation Another gene, CCN1, has been implicated as well; losing just one copy of Ccn1 in mice was enough to cause persistent ostium primum ASDs in roughly 20% of adults and delayed ventricular septum formation during embryonic life.9PubMed Central. The matricellular protein CCN1 is essential for cardiac development
Environmental factors layer onto genetic susceptibility. A case-control study found that maternal alcohol consumption during the first trimester roughly doubled the odds of atrial septal defect in the offspring.10PubMed. Risk factors for atrial septal defect This is consistent with the broader picture that early cardiac development is especially vulnerable to teratogenic exposure because so many structures, including the septum primum, are forming and fusing during that window.
When the Foramen Ovale Closes Too Early
Most clinical attention focuses on the septum primum failing to close properly, but the opposite problem also exists: premature restriction or closure of the foramen ovale in utero. If the flap valve seals before birth, the fetal right heart loses its outlet for placental blood into the left atrium. Right atrial and right ventricular pressures climb, the right side of the heart dilates, and the tricuspid valve may begin leaking. In severe cases this can progress to heart failure, fluid accumulation around the fetal heart, and hydrops fetalis.
Premature foramen ovale restriction is rare in fetuses with structurally normal hearts, and most affected pregnancies still result in a live birth with a good outcome. However, there is an association between restriction or closure and serious complications including prematurity, hydrops, and neonatal death, particularly when complete closure occurs.11PubMed. Isolated premature restriction or closure of foramen ovale in fetuses: Echocardiographic characteristics and outcome The most severe outcomes include arrhythmias, heart failure, and hydrops fetalis.12PubMed. In Utero Diagnosis of Premature Closure of the Foramen Ovale: A Retrospective Case Series Analyzing Neonatal Interventions and Maternal Factors Some cases have been linked to maternal use of nonsteroidal anti-inflammatory drugs (NSAIDs), which can promote ductal and foramen ovale constriction in the fetus, though many cases have no identifiable cause.
Atrial Septal Aneurysm
Sometimes the septum primum doesn’t just stay put or refuse to fuse; it becomes excessively mobile, billowing back and forth between the atria with each cardiac cycle. When the excursion exceeds a threshold (typically 10 mm or more), the bulging tissue is classified as an atrial septal aneurysm (ASA). Despite the alarming name, an ASA is not at immediate risk of rupturing the way an arterial aneurysm might. The concern is more subtle: a hypermobile septum primum often coexists with a PFO, and the combination of the two appears to carry a higher risk of paradoxical embolism and stroke than either finding alone. Atrial septal aneurysms also provide a window into atrial function and pressure differences, and they have been associated with arrhythmias and migraine with aura.13PubMed. Atrial Septal Aneurysms – A Clinically Relevant Enigma?
Challenges in Catheter-Based Repair
For patients who need their ASD or PFO closed, catheter-based (transcatheter) closure has become a common alternative to open-heart surgery. A closure device is threaded through a vein and deployed across the defect, where it grips the surrounding tissue. The septum primum’s anatomy directly affects whether this approach will work. When the septum primum is malattached, meaning it connects to the atrial wall in an abnormal position, the device may not have enough tissue to anchor against. A study of transcatheter ASD closures found that a malattached septum primum, along with deficient retroaortic and inferior rims, was independently associated with procedural failure.14PubMed. Malattached septum primum and deficient septal rim predict unsuccessful transcatheter closure of atrial communications Identifying these anatomical features on echocardiography beforehand helps interventional cardiologists decide whether catheter-based repair is feasible or whether the patient should be referred for surgical closure instead.
An Evolutionary Quirk of Placental Mammals
One of the more surprising facts about the septum primum is how unusual the human version of atrial septation is compared with other animals. A complete division of the atrial chamber exists in many amphibians and all amniotes (reptiles, birds, and mammals). But only in placental (eutherian) mammals does full atrial septation require a second septum. In reptiles and birds, the secondary perforations that appear in the septum primum’s myocardial wall remain as small individual holes rather than merging into one large secondary foramen. After hatching or birth, those small perforations are sealed by local tissue growth. In placental mammals, by contrast, the perforations coalesce into the single large ostium secundum, which is then covered by the septum secundum acting as a rigid frame and the septum primum acting as a flap valve.15PubMed Central. Evolution and Development of the Atrial Septum
Why the extra complexity? The leading hypothesis is that the large foramen ovale of placental mammals allows a much greater volume of oxygen-rich blood to cross from the right atrium to the left atrium during fetal life, supporting the growth of the unusually large offspring that characterize placental mammals compared with egg-laying or marsupial species. The tradeoff is that this design is more error-prone: whenever two separate structures must fuse after birth, the opportunity for incomplete fusion rises. That may be why PFO is so common in humans. In a sense, the septum primum’s imperfect closure rate is the evolutionary price of being born large-brained and well-developed rather than hatching from an egg.
The most primitive version of an atrial septum appears in lungfishes, which share many of the same tissue types found in the mammalian septum primum: a muscular core capped by mesenchymal tissue at its growing edge, reminiscent of the mammalian primary septum before it closes the ostium primum. The deep conservation of these building blocks across hundreds of millions of years of vertebrate evolution underscores how fundamental atrial septation is to the transition from single-circuit to double-circuit circulation.