Systemic Lamination of the Retroperitoneal Fascia Driven by the Anomalous Poisson Effect: In Vivo Evidence from Congenital Renal Agenesis and the Mechanical Phase Transition at 20 Fetal Weeks

For decades, the prevailing interpretation, originating with Tobin (1944) and firmly supported by Benjamin and Tobin (1951), has been that “without a kidney, the renal fascia does not form.” However, this premise was based on specific anatomical specimens lacking both the kidney and the adrenal gland, and has never been systematically verified through modern diagnostic imaging. In this study, we utilized congenital unilateral renal agenesis as a natural subtraction experiment to re-evaluate the mechanical model of retroperitoneal fascial lamination. In all three adult cases identified from 5,509 non-contrast CT scans, a distinct fascial plane corresponding to the parietal lamina (outer layer) of the posterior renal fascia was invariably preserved, despite the lifelong absence of the kidney. A uniform thinning was observed on the agenesis side (mean 1.52 mm vs. 1.85 mm), suggesting the selective loss of the organ-dependent “inner lamina” and the autonomous formation of the system-derived “outer lamina.” Integrating fetal embryology, materials science, and mechanical modeling, this framework aligns with the early truncal stiffening (evolutionary front-loading) and the extracellular matrix (ECM) material phase transition occurring around 20 fetal weeks. These events induce a multiaxial tension field and orthogonal Poisson compression, which may drive the macroscopic lamination of the retroperitoneal fascia. We outline a conceptual “two-stage alignment model” consisting of passive Poisson compression followed by active mechanotransduction, suggesting that retroperitoneal fascial formation cannot be explained solely by organ dependency. Furthermore, we present a biomechanical “causal loop,” wherein the formed planar fascial sheets undergo a functional turn into active tensegrity cables that distribute and transmit the gravitational loads encountered during postnatal erect bipedalism.