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 our new 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 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 custom fiber-network analyses. We tested our null hypothesis that the Poisson's function was greater than zero, i.e. ν^Green>0. Across all specimens, the average median ν^Green was -0.842, with regional values of -0.539, -0.663, and -1.322 for the colonic, intermediate, and rectal segments, respectively. Using a one-sided, one-sample t-test with a significance level (α) of 0.05, the total sample size (n=30) achieves >99.99% power to reject the null hypothesis. Regional analyses (n=10 per region) achieve powers of 89.66%, 96.73%, and 99.55% for the colonic, intermediate, and rectal regions, respectively, to reject the null hypothesis. Corresponding analyses of the nerve fibers across nine specimens revealed an average median stretch ratio of 1.063, indicating approximately six percent elongation, with substantial heterogeneity driven by fiber orientation. Our results confirm that the colorectum exhibits auxetic mechanics, and those mechanics transmit to embedded sensory nerve fibers, thus enhancing our understanding of mechanotransduction (e.g. mechanical and nociceptive signaling) and informing both development of treatment targets for visceral pain and design of bioinspired auxetic materials.