Poisson Compression Theory (PCT) for Anisotropic Fibrous Layer Formation

[Background: The Spatial Paradox and the Return of Causality] This study restructures the morphogenesis of connective tissue through a three-tiered causal hierarchy: "Boundary Conditions → Physics → Biology," and proposes the "Poisson Compression Theory (PCT)," which positions anomalous Poisson compression and poroelastic fluid efflux (syneresis) as the primary physical engines. The major theoretical limitation of the conventional fibroblast-centric paradigm lies in the spatial paradox of assigning a "global geometric design" to local cells. While cells can sense local strain, they entirely lack the biological capacity to recognize global coordinate systems, navigate multi-layered intersection angles, or map the macroscopic topography of body-wide tension fields. To resolve this spatial paradox, this study returns the causal structure of morphogenesis to a physical hierarchy. [Core of PCT: Two Physical Engines] The core of PCT postulates that the geometric mismatch between fetal volumetric expansion (V∝r3) and surface area expansion (A∝r2) generates a universal "growth-induced tension." When this tension is confined within closed boundary conditions (a mechanical frame), the following two physical engines are sequentially activated: Anomalous Poisson Compression (νeff≥1.5): Instantaneously and passively aligns collagen and fibroblasts along the principal strain axis. Poroelastic Syneresis (Fluid Efflux): Induces irreversible Z-axis flattening (2D geometric projection). This two-phase physical process demonstrates that the geometry of the tension field determines morphology prior to active cellular design (biological stabilization). [Morphological Bifurcation by Boundary Conditions and Natural Experiments] Boundary conditions strictly dictate this morphology. Specifically, even with identical physical engines, the dimensionality of the tension field generates different structures: uniaxial tension produces 1D cord-like structures (ligaments and tendons), whereas biaxial tension generates 2D planar sheets (fasciae and aponeuroses). Morphological bifurcation is completely determined by the stress tensor of the tension field, rather than by cellular programming. This fact is supported by natural experiments, such as the autonomous formation of the "anterior leaf (anterior renal fascia)" in congenital unilateral renal agenesis. The phenomenon where the fascia forms despite the lifelong absence of the kidney proves "Local Field Independence," demonstrating that morphology is determined not by local organ crowding, but autonomously by the global tension field created by the pelvis, vertebral column, and peritoneal sac. [Evolutionary Compensation and Systematic Lamination] Furthermore, PCT highlights an evolutionary compensatory mechanism regarding the uniquely human iliac flare, which exhibits delayed ossification associated with bipedalism. To compensate for this structural vulnerability, the fetus accelerates the maturation of connective tissue, establishing the fascia as an outer shell earlier in gestation through "complementary heterochrony." As a result, growth-induced tension (V/A∝r) amplifies mid-gestational Poisson compression, establishing the systematic lamination of the fascia. [Abstract Summary] By identifying anomalous Poisson compression and poroelastic syneresis as the primary engines and positioning the geometry of growth (r3−r2) as the primordial fuel for morphogenesis, PCT redefines fascial formation through a rigorous physical causal hierarchy.