Comparison of enteric neuronal morphology as demonstrated by Dil-tracing under different tissue-handling conditions

The aim of this study was to determine locations and morphologies of enteric neurons innervating the small intestinal mucosa of the pig after application of the carbocyanine tracer Dil onto a single villus. The tissue was processed in two ways: incubation (1) of fixed material (postmortem tracing) for several months and (2) of living specimens within organotypic culture in vitro for several days (supravital tracing). In both procedures Dil-labelled neurons were found in the three ganglionated plexuses, the internal and external submucous plexus as well as the myenteric plexus. Postmortem tracing revealed different neuronal morphologies. Adendritic type II neurons were present in all three plexuses, type IV neurons with short, scarcely branched, polarly emerging dendrites were mainly found in the myenteric plexus and small dendritic neurons were mainly present in the internal submucous plexus. The latter may correspond to minineurons hitherto described only immunohistochemically. Tracing within tissue culture showed somata of neurons and, partly, proximal segments of processes to be labelled. Subsequent immunohistochemistry using general neuronal markers revealed some neurons to be adendritic type II neurons. Visualization of dendrites was less clear, hampering an accurate morphological classification of dendritic neurons. Our results suggest that neurons of all ganglionated enteric nerve plexuses of the pig participate in the innervation of the mucosa, and that postmortem tracing revealed enteric neuronal morphology more clearly than supravital tracing. Since the former method cannot be applied for deciphering the chemical coding of enteric neurons, combination of both methods will extend our knowledge of the morphological substrate for the intrinsic neuronal microcircuits in the gastrointestinal tract.