Origin and morphology of nerve fibers the aganglionic colon of the lethal spotted (ls/ls) mutant mouse
Authors: Payette Robert F. , Tennyson Virginia M. , Pham Tuan Duc , Mawe Gary M. , Pomeranz Howard D. , Rothman Taube P. , Gershon Michael D. 
Published: January 2005

The lethal spotted mutant mouse (ls/ls) develops congenital megacolon because of the absence of ganglia in the terminal colon. This aganglionosis results from a failure of neural crest cells to colonize this area during fetal life. We have postulated that the microenvironment of the aganglionic segment of bowel is abnormal. Our hypothesis suggests that this abnormal enteric microenvironment fosters the sprouting of neuritic processes. We further propose that neural and glial precursors cease to migrate once they have extended their definitive processes. As a result, the area distal to the site where neurite extension is favored does not become colonized by neural or glial precursors. A prediction of this hypothesis is that the aganglionic tissue should be innervated by axons from neurons located both in the more proximal ganglionated bowel and in ganglia located outside the gut. Neurons and their processes in control and ls/ls terminal gut were located by the histochemical demonstration of acetylcholinesterase (AChE) activity and their structure was classified as intrinsic (enteric) or extrinsic in type by electron microscopy. In ls/ls mice the submucosal plexus was much more severely affected than the myenteric plexus. No submucosal ganglia were found within 30 mm of the anus. In contrast, myenteric ganglia extended to within 4 mm of the anus on the mesenteric side of the gut and to within 15 mm on the antimesenteric side. Rostral to the areas that were absolutely aganglionic, both plexuses were hypoganglionic, especially the submucosal plexus, which was hypoganglionic throughout the entire colon. Both the aganglionic and caudal hypoganglionic zones of the ls/ls bowel were penetrated by large nerve trunks that had the ultrastructural characteristics of extra‐enteric peripheral nerve. Unusual ganglia, outside the enteric musculature in the adventitia of the colon, were connected to these trunks. The location of the cell bodies of origin of the nerve fibers in the terminal colon of control mice and in the aganglionic segment of the bowel in ls/ls mice was determined by following the retrograde transport of tracers injected as close as possible to the anus. An extrinsic innervation originating from the inferior mesenteric ganglion and dorsal root ganglia (L6‐S1) was found in both types of animal. In control but not ls/ls mice retrograde labeling was also observed in the sacral parasympathetic nucleus of the spinal cord. In addition, neuritic processes were traced to neurons in myenteric ganglia. In control mice, these labeled neurons were present in ganglia within the injection site as well as in bowel rostral and caudal to it. In ls/ls mice, labeled neurons were only observed in the more proximal ganglionated region of the gut. Measurements of the maximal lengths of enteric axons indicated that ls/ls axons were slightly shorter than those of control animals and probably did not penetrate distally as far as the anus except possibly on the mesenteric side of the gut. These observations support the idea that the microenvironment of the aganglionic bowel of ls/ls mice, however poor it microenvironment of the neural and glial precursors, is a good substrate for axonal growth. Neither intrinsic nor extrinsic neurites appear to be excluded from this region and extrinsic neurites appear to be attracted to the tissue in greater than normal abundance.