Colorectum and Embedded Networks of Nerve Fibers Present Auxetic Responses During Uniaxial Circumferential Extension

Understanding the multiscale mechanics of the colorectum is essential for uncovering the mechanotransductive pathways underlying visceral nociception. Intraluminal distension of the large intestine reliably evokes pain in disorders of gut-brain interaction (DGBIs), yet the tissue-level and nerve fiber-level responses to mechanical loading remain poorly defined. Here, we present results from a novel biomechanical testing framework that integrates uniaxial circumferential extension with high-resolution optical imaging to quantify deformation in both bulk colorectal tissue and embedded sensory nerve fibers. We tested intact, cylindrical colorectal segments from mice using a custom 3-D-printed chamber with intraluminal stainless-steel rods to apply circumferential stretch while maintaining a planar imaging field. We measured bulk-tissue deformation via Digital Image Correlation (DIC), while we assessed stretch in nerve fibers through fluorescence imaging of VGLUT2-labeled afferents analyzed using a custom fiber-network analyses. Across specimens, we observed a consistent auxetic response-characterized by positive axial strain during circumferential extension-at both the macroscale and microscale. Five out of six colorectal specimens exhibited positive axial Green-Lagrange strain (Exx), with an average median Exx of 0.0177, during circumferential extension generating an average median Eyy of 0.1273. Nerve fiber analysis across nine specimens revealed an average median stretch ratio of 1.0631, indicating 6.31% elongation, with substantial heterogeneity driven by fiber orientation. These findings demonstrate that the colorectum and its embedded network of nerve fibers exhibit auxetic behavior, a property that may amplify mechanical signaling and influence nociceptive signaling. Our methods and results provide foundational insight into structure-function relationships of colorectum and inform design of bioinspired auxetic materials.