A 24-week-old male fetus is evaluated by prenatal ultrasound and found to have an omphalocele. Histological examination of the defect reveals that the fascial defect is covered by a layer of peritoneum and amnion but lacks muscular tissue. The embryologist explains that this defect resulted from incomplete fusion of the lateral body wall folds during weeks 4–8 of gestation. Which of the following embryological structures failed to migrate or fuse properly to result in this presentation?

Explanation

## Embryological Basis of Omphalocele An omphalocele is a ventral midline defect of the abdominal wall caused by **incomplete fusion of the somatopleure** (parietal layer of lateral plate mesoderm) during weeks 4–8 of gestation. ### Normal Development: - The **somatopleure** gives rise to the parietal peritoneum, transversus abdominis, internal and external oblique muscles, and rectus abdominis. - During weeks 4–8, the lateral body wall folds (derived from somatopleure) migrate medially and fuse at the ventral midline. - This fusion encloses the coelomic cavity and forms the abdominal wall. ### Pathophysiology of Omphalocele: - **Incomplete fusion** of the lateral somatopleural folds leaves a defect at the umbilicus. - The defect is covered by **peritoneum (from splanchnopleure) and amnion** but **lacks muscular tissue** (which normally comes from somatopleure). - The fascial defect may contain bowel, liver, or other viscera. ### Key Distinction: - **Gastroschisis** (defect lateral to umbilicus, no peritoneal/amniotic covering) results from disruption of the **omphalomesenteric artery** and vascular insufficiency, NOT from mesodermal fusion failure. - **Omphalocele** (defect at umbilicus, peritoneal/amniotic covering) results from somatopleural fusion failure. **Clinical Pearl:** The presence of peritoneal and amniotic covering (as stated in the stem) is pathognomonic for omphalocele, confirming the diagnosis and pointing to somatopleural embryological failure. **High-Yield:** Somatopleure = parietal layer = body wall musculature. Splanchnopleure = visceral layer = gut tube and its mesenteries.