Which embryological structure gives rise to the membranous interventricular septum?

Explanation

## Embryological Origin of the Interventricular Septum ### Muscular vs. Membranous Components The interventricular septum (IVS) develops from two distinct sources: | Component | Origin | Timing | Clinical Significance | |-----------|--------|--------|----------------------| | **Muscular IVS** | Myocardial ingrowth from primitive ventricle | Weeks 4–7 | Forms bulk of septum | | **Membranous IVS** | Endocardial cushions + aorticopulmonary septum | Weeks 7–9 | Smallest portion; site of VSD | **Key Point:** The membranous interventricular septum is derived from **endocardial cushions** (which also contribute to AV valve formation and atrial septation). It forms the superior and posterior portion of the IVS, just below the aortic valve. **High-Yield:** Defects in endocardial cushion development → membranous VSD (most common type of VSD, ~80% of cases). This is why endocardial cushion abnormalities are associated with both septal defects and AV valve anomalies (e.g., cleft mitral valve in Down syndrome). **Clinical Pearl:** The membranous septum is the last part of the IVS to develop (weeks 7–9); premature arrest of growth at this stage results in a small restrictive membranous VSD, which may close spontaneously in infancy. ### Embryological Timeline 1. **Week 4:** Myocardial proliferation begins forming muscular IVS 2. **Weeks 5–7:** Muscular IVS grows toward endocardial cushions 3. **Weeks 7–9:** Endocardial cushion tissue + aorticopulmonary septum fuse with muscular IVS to complete the septum 4. **Week 9:** IVS is complete; foramen ovale remains as normal ASD equivalent **Mnemonic:** **ECAD** = **E**ndocardial **C**ushions form **A**V valves **D**evelop (and also membranous IVS).