Load-bearing function of the colorectal submucosa and its relevance to visceral nociception elicited by mechanical stretch.

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 this tissue into inner and outer composite layers. The inner composite consists of the mucosa and submucosa, whereas 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 that the inner composite is slightly stiffer in the axial direction, whereas 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° to the axial direction. Submucosal thickness measured by SHG showed no difference from proximal to distal colorectum under the 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, which likely encode tissue-injurious mechanical distension. Visceral pain from distal colon and rectum (colorectum) is usually elicited from mechanical distension/stretch, rather than from heating, cutting, or pinching, which usually evoke pain from the skin. We conducted layer-separated biomechanical tests on mouse colorectum and identified an unexpected role of submucosa as the load-bearing structure of the colorectum. Outcomes of this study will focus attention on sensory nerve endings in the submucosa that likely encode tissue-injurious distension/stretch to cause visceral pain.