All of the following statements regarding the endocardial cushions and their derivatives are true EXCEPT:

Explanation

## Endocardial Cushions: Origin and Fate ### Embryological Origin — NOT Neural Crest Alone **Key Point:** Endocardial cushions are derived from **splanchnic mesoderm and neural crest**, NOT exclusively from neural crest. This is a critical distinction in cardiac embryology. | Component | Primary Origin | Secondary Contribution | | --- | --- | --- | | Endocardial cushion mesenchyme | Splanchnic mesoderm (epicardial origin) | Neural crest (minor) | | Valve leaflets | Endocardial cushion tissue | Fibroblasts from NC | | Fibrous skeleton | Endocardial cushion + mesoderm | Neural crest connective tissue | ### Correct Statements About Endocardial Cushion Derivatives **High-Yield:** The endocardial cushions (dorsal and ventral) appear at week 4 and undergo the following fates: 1. **Atrioventricular canal septation** — cushions fuse to divide the common AV canal into right (tricuspid) and left (mitral) orifices ✓ 2. **Dorsal cushion → anterior (septal) leaflet of mitral valve** ✓ 3. **Ventral cushion → septal leaflet of tricuspid valve** ✓ 4. **Membranous IVS contribution** — upper portion of membranous septum forms from fused cushion tissue ✓ 5. **Atrial septation** — contribute to formation of the septum primum ✓ ### Why the Marked Answer is Correct **Warning:** Endocardial cushion mesenchyme is **NOT derived entirely from neural crest**. The primary source is **splanchnic mesoderm** (epicardial/visceral mesoderm). Neural crest contributes supporting fibroblasts and connective tissue but is NOT the sole origin. **Clinical Pearl:** Endocardial cushion defects (e.g., ostium primum ASD, complete AV canal defect) arise from abnormal mesenchymal proliferation and fusion, often associated with trisomy 21 — a mesodermal developmental anomaly, not a neural crest migration defect. ### Mnemonic **ECU = Endocardial Cushion Unites:** **E**ndocardial cushion (splanchnic mesoderm origin), **C**ontributes to valves and septa, **U**nites AV canal into left and right orifices.