Development of interatrial septum in flow chart with diagrams
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I now have comprehensive content and two key diagrams. Let me compile the complete flowchart-style answer with diagrams.
Development of the Interatrial Septum
Sources: Langman's Medical Embryology, 14th ed. | The Developing Human, Clinically Oriented Embryology | Robbins & Cotran Pathologic Basis of Disease
Overview
The interatrial septum forms between weeks 4-7 of embryonic development through the sequential growth of two membranes - the septum primum and septum secundum - and their interaction with the endocardial cushions.
Flowchart of Development
┌─────────────────────────────────────────────────────────────────────────┐
│ COMMON ATRIUM (single undivided chamber, ~Week 4) │
└───────────────────────────┬─────────────────────────────────────────────┘
│
▼
┌─────────────────────────────────────────────────────────────────────────┐
│ STEP 1: SEPTUM PRIMUM FORMS (~Day 28 / End of Week 4) │
│ │
│ • Sickle-shaped (crescent) crest grows DOWN from roof of atrium │
│ • Two limbs extend toward endocardial cushions in AV canal │
│ • Gap between lower rim of septum primum and endocardial │
│ cushions = OSTIUM PRIMUM (foramen primum) │
│ • Allows right-to-left blood flow during fetal life │
└───────────────────────────┬─────────────────────────────────────────────┘
│
▼
┌─────────────────────────────────────────────────────────────────────────┐
│ STEP 2: OSTIUM PRIMUM CLOSES + OSTIUM SECUNDUM FORMS (~Day 33, 9mm) │
│ │
│ • Extensions of superior and inferior endocardial cushions grow │
│ along the edge of septum primum → OSTIUM PRIMUM CLOSES │
│ • BEFORE closure is complete: │
│ - Programmed cell death (apoptosis) creates perforations in │
│ the UPPER portion of septum primum │
│ - Perforations coalesce → OSTIUM SECUNDUM (foramen secundum) │
│ • Ostium secundum ensures CONTINUED free right-to-left blood flow │
└───────────────────────────┬─────────────────────────────────────────────┘
│
▼
┌─────────────────────────────────────────────────────────────────────────┐
│ STEP 3: SEPTUM SECUNDUM GROWS (~Day 37, 14mm / ~Week 5-6) │
│ │
│ • New crescent-shaped fold appears to the RIGHT of septum primum │
│ as the right atrium expands by incorporating the sinus horn │
│ • Septum secundum grows downward (like a window shade) │
│ • Its anterior limb extends to the AV canal septum │
│ • Left venous valve + septum spurium fuse with right side of │
│ septum secundum │
│ • NEVER forms a complete partition - leaves a gap │
└───────────────────────────┬─────────────────────────────────────────────┘
│
▼
┌─────────────────────────────────────────────────────────────────────────┐
│ STEP 4: FORAMEN OVALE ESTABLISHED (Fetal Life) │
│ │
│ • Septum secundum overlaps but does NOT cover the ostium secundum │
│ completely → opening = FORAMEN OVALE │
│ • Upper part of septum primum gradually disappears │
│ • Remaining inferior part of septum primum = │
│ VALVE OF THE FORAMEN OVALE │
│ • Blood flows through an obliquely elongated cleft: │
│ RIGHT atrium → LEFT atrium (right-to-left shunt) │
│ • This is NORMAL in fetal life (bypasses non-functional lungs) │
└───────────────────────────┬─────────────────────────────────────────────┘
│
▼
┌─────────────────────────────────────────────────────────────────────────┐
│ STEP 5: FUNCTIONAL CLOSURE AT BIRTH │
│ │
│ • Lungs expand → pulmonary vascular resistance falls │
│ • Pulmonary blood flow ↑ → left atrial pressure RISES above right │
│ • Valve of foramen ovale is PRESSED against septum secundum │
│ • Foramen ovale functionally sealed → no more right-to-left shunt │
└───────────────────────────┬─────────────────────────────────────────────┘
│
▼
┌─────────────────────────────────────────────────────────────────────────┐
│ STEP 6: ANATOMIC CLOSURE (by ~3rd postnatal month) │
│ │
│ • Tissue proliferation + adhesion of septum primum to left margin │
│ of septum secundum → permanent fusion │
│ • Septum primum → floor of OVAL FOSSA (fossa ovalis) │
│ • Free edge of septum secundum → border of oval fossa │
│ (LIMBUS FOSSAE OVALIS) in adult heart │
└─────────────────────────────────────────────────────────────────────────┘
Key Developmental Stages - Diagram
The diagram below (Langman's Medical Embryology, Fig. 13.16) shows all stages A through G:
Guide to the diagram:
| Panel | Stage | Key Features |
|---|---|---|
| A (30 days, 6mm) | Septum primum growing down | Ostium primum open; endocardial cushions forming |
| B (same stage, right view) | Apoptosis begins in upper septum primum | Region of cell death visible; ostium primum still present |
| C (33 days, 9mm) | Ostium secundum formed; septum secundum begins | Endocardial cushions fused closing ostium primum |
| D (same stage, right view) | Both septa visible | Ostium secundum in septum primum; septum secundum growing |
| E (37 days, 14mm) | Septum secundum overlapping ostium secundum | Foramen ovale established |
| F (Newborn) | Post-birth anatomy | Foramen ovale, membranous IVS, muscular IVS |
| G (Newborn, right view) | Final atrial septum | Septum secundum, valve of foramen ovale (= septum primum), valves of IVC and coronary sinus |
Adult Anatomy Diagram (Normal vs. Patent Foramen Ovale)
The diagram below (The Developing Human, Fig. 13.25) shows the normal postnatal interatrial septum (A/A1) versus probe-patent foramen ovale (B/B1):
In the normal adult heart:
- The oval fossa (fossa ovalis) = depression on right atrial wall
- Its floor = septum primum (thin, fibrous)
- Its border (limbus fossae ovalis) = free edge of septum secundum
Summary Table: Structures and Their Adult Derivatives
| Fetal Structure | Role During Fetal Life | Adult Derivative |
|---|---|---|
| Septum primum | First partition; forms valve of foramen ovale | Floor of fossa ovalis |
| Ostium primum | Transient gap (right-to-left flow) | Obliterated by endocardial cushions |
| Ostium secundum | Formed by apoptosis; maintains right-to-left flow | Covered by septum secundum |
| Septum secundum | Second (thicker) partition growing from right | Border of fossa ovalis (limbus) |
| Foramen ovale | Opening in septum secundum; fetal right-to-left shunt | Fossa ovalis after closure |
| Valve of foramen ovale | Remnant of lower septum primum; opens/closes flap | Fused into floor of fossa ovalis |
Clinical Correlations
Atrial Septal Defects (ASDs)
(Robbins & Cotran, p. 504)
| Type | % of ASDs | Embryological Basis |
|---|---|---|
| Secundum ASD (most common) | ~90% | Deficient septum secundum formation OR excessive resorption of septum primum near center of atrial septum |
| Primum ASD | ~5% | Failure of endocardial cushions to close ostium primum; often associated with AV valve abnormalities |
| Sinus venosus defect | ~5% | Abnormal absorption of sinus venosus near SVC entrance; may have anomalous pulmonary venous return |
Patent Foramen Ovale (PFO)
- Occurs in ~20% of the population (Langman's)
- Incomplete fusion of septum primum to septum secundum
- Leaves a narrow oblique cleft - a probe can pass between the two septa
- Generally no intracardiac shunting if the opening is small (left atrial pressure keeps it closed in normal adults)
- Clinical significance: potential pathway for paradoxical embolism (cryptogenic stroke)
Key Mnemonic
"P comes before S, O comes before O"
- Primum appears before Secundum
- Ostium primum closes, then Ostium secundum opens
- Think of it as a relay: each structure hands off to the next before it disappears
- Langman's Medical Embryology, 14th ed., pp. 209-211
- The Developing Human, Clinically Oriented Embryology, pp. 828-831
- Robbins & Cotran Pathologic Basis of Disease, p. 504
- Medical Physiology (Boron & Boulpaep), p. 1690-91
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