Fetal Week 20 as the Mechanical Turning Point of Retroperitoneal Fascial Lamination: A Poisson-Effect Framework

This study presents a theoretical mechanobiological model explaining the multilaminated architecture of retroperitoneal fasciae. Classical peritoneal fusion theories cannot account for either these organized laminae or the 10‑week delay between early visceral fixation and definitive fascial formation. We propose that early localized tension at 10–12 gestational weeks forms the inner renal fascial layer, whereas a systemic tension field emerging around 20 weeks—driven by axial skeletal ossification, pelvic expansion, and exponential volumetric growth—induces orthogonal Poisson‑effect compression, poroelastic fluid exudation, and LOX‑mediated cross‑linking to generate the laminated outer layer. To illustrate this framework, we examined a pure clinical cohort of adult renal vacancy (n=3) from 5,509 CT scans. Despite lifelong absence of the kidney, a continuous outer fascial layer persisted, indicating that its formation is tension‑driven rather than organ‑dependent. This natural subtraction phenomenon resolves the long‑standing discrepancy between classical dissection and modern imaging and supports a systemic mechanobiological origin for retroperitoneal fascial lamination.