Search for: All records

Many lower gastrointestinal diseases are associated with altered mechanical movement and deformation of the large intestine, i.e., the colon and rectum. The leading reason for patients’ visits to gastrointestinal clinics is visceral pain, which is reliably evoked by mechanical distension rather than non-mechanical stimuli such as inflammation or heating. The macroscopic biomechanics of the large intestine were characterized by mechanical tests and the microscopic by imaging the load-bearing constituents, i.e., intestinal collagen and muscle fibers. Regions with high mechanical stresses in the large intestine (submucosa and muscularis propria) coincide with locations of submucosal and myenteric neural plexuses, indicating a functional interaction between intestinal structural biomechanics and enteric neurons. In this review, we systematically summarized experimental evidence on the macro- and micro-scale biomechanics of the colon and rectum in both health and disease. We reviewed the heterogeneous mechanical properties of the colon and rectum and surveyed the imaging methods applied to characterize collagen fibers in the intestinal wall. We also discussed the presence of extrinsic and intrinsic neural tissues within different layers of the colon and rectum. This review provides a foundation for further advancements in intestinal biomechanics by synergistically studying the interplay between tissue biomechanics and enteric neurons. 

Mechanical distension beyond a particular threshold evokes visceral pain from distal colon and rectum (colorectum) and thus biomechanics plays a central role in visceral nociception. In this study we focused on the layered structure of the colorectum through the wall thickness and determined the biomechanical properties of layer-separated colorectal tissue. We harvested the distal 30 mm of mouse colorectum and dissected into inner and outer composite layers. The inner composite consists of the mucosa and submucosa while the outer composite includes the muscular layers and serosa. We divided each composite axially into three 10 mm-long segments and conducted biaxial mechanical extension tests and opening-angle measurements for each tissue segment. In addition, we quantified the thickness of the rich collagen network in the submucosa by nonlinear imaging via second harmonic generation (SHG). Our results reveal the inner composite is slightly stiffer in the axial direction while the outer composite is stiffer circumferentially. The stiffness of the inner composite in the axial direction is about twice that in the circumferential direction, consistent with the orientations of collagen fibers in the submucosa approximately ±30 degrees to the axial direction. Submucosal thickness measured by SHG showed no difference from proximal to distal colorectum under load-free condition, which likely contributes to the comparable tension stiffness of the inner composite along the colorectum. This, in turn, strongly indicates the submucosa as the load-bearing structure of the colorectum. This further implies nociceptive roles for the colorectal afferent endings in the submucosa that likely encode tissue-injurious mechanical distension.