All of the following structures are derived from the endocardial cushions EXCEPT:

Explanation

## Embryological Origin of Cardiac Valves and Septa ### Endocardial Cushion Derivatives **Key Point:** The endocardial cushions are mesenchymal proliferations that appear in the atrioventricular canal and conus arteriosus during weeks 4–7 of development. They give rise to critical septal and valvular structures. ### Structures Derived from Endocardial Cushions | Structure | Origin | Timing | | --- | --- | --- | | Mitral (bicuspid) valve | Endocardial cushions + myocardium | Week 5–8 | | Tricuspid valve | Endocardial cushions + myocardium | Week 5–8 | | Membranous interventricular septum | Endocardial cushions | Week 5–7 | | Atrioventricular septum | Endocardial cushions | Week 4–5 | | Anterior leaflet of mitral valve | Dorsal cushion | Week 5–8 | | Septal leaflet of tricuspid valve | Ventral cushion | Week 5–8 | ### Origin of the Aortic Valve **High-Yield:** The aortic valve is derived from **neural crest cells** and **splanchnic mesoderm** of the conus arteriosus, NOT from endocardial cushions. The semilunar valves (aortic and pulmonary) develop from the truncus arteriosus and conus arteriosus, which undergo truncal septation by the spiral septum. ### Clinical Pearl **Warning:** Endocardial cushion defects (ECDs) result in failure of cushion tissue to fuse properly, leading to: - Ostium primum atrial septal defect (ASD) - Inlet ventricular septal defect (VSD) - Cleft mitral valve - Cleft tricuspid valve Because the aortic valve is NOT derived from endocardial cushions, it is **NOT affected** in classic endocardial cushion defects. ### Mnemonic **CUSHION structures:** **C**left mitral, **U**nder-developed tricuspid, **S**eptal defects (ASD/VSD), **H**ole in atrioventricular septum, **I**nlet VSD, **O**stium primum, **N**ot the aortic valve. **Key Point:** Aortic and pulmonary valves = semilunar valves from truncus/conus; AV valves (mitral, tricuspid) = endocardial cushion origin.