No abstract

In the present study, a combination of immunohistochemistry and retrograde 1,1`‐didodecyl‐3,3,3`,3`‐tetramethylindocarbocyanine perchlorate (DiI) tracing was used to unravel the morphology, distribution, and neurochemical coding of submucous and myenteric neurons with axonal projections to the mucosa of the porcine small intestine. The majority of traced neurons was located in the inner submucous plexus (ISP; 78%), whereas the remaining part was distributed between the outer submucous plexus (OSP; 10%) and myenteric plexus (MP; 12%). Among these traced neurons, some distinct neuronal populations could be distinguished according to their morphologic and neurochemical properties. In the ISP, several types of traced neurons were detected: 1) morphologic type II neurons expressing choline acetyltransferase (ChAT) immunoreactivity, calcitonin gene‐related peptide (CGRP) immunoreactivity, and substance P (SP) immunoreactivity; 2) ChAT/SP‐immunoreactive (‐IR) small neurons; 3) vasoactive intestinal polypeptide (VIP) ‐IR small neurons; and 4) multidendritic ChAT/somatostatin (SOM) ‐IR neurons. The traced neuronal populations of the OSP and MP were similar to each other. In both plexuses, the following DiI‐labelled neurons were found: 1) ChAT/CGRP/(SP)‐IR type II neurons; 2) multidendritic ChAT/SP‐IR neurons; and 3) multidendritic ChAT/SOM‐IR neurons. Comparison of the present findings with previously obtained data concerning the mucosal innervation pattern of the intestine of small mammals, revealed significant species differences with respect to the morphologic and neurochemical features of the involved enteric neuronal classes. Although not identical, a closer resemblance between pig and human enteric nervous system seems to be at hand, as far as the anatomic organization and the presence of neurochemically identified neuronal subtypes within the enteric nervous system are concerned. J. Comp. Neurol. 421:429–436, 2000. © 2000 Wiley‐Liss, Inc.

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.

Neuroendocrine-specific protein (NSP) reticulons are expressed in neural and neuroendocrine tissues and cell cultures derived therefrom, while most other cell types lack NSP-reticulons. Three major subtypes have been identified so far, designated NSP-A, NSP-B, and NSP-C. We have investigated the correlation between the degree of neuronal differentiation, determined by morphological and biochemical criteria, and NSP-reticulon subtype expression. For this purpose, several human neuroblastoma cell lines, exhibiting different degrees of neuronal differentiation, were examined immuno(cyto)chemically. It became obvious that the expression of NSP-C, as detected by immunofluorescence microscopy and Western blotting, is most prominent in cell lines with a high degree of neuronal differentiation, such as LA-N-5. Such highly differentiated cells also express other neural and neuroendocrine markers, such as neural cell adhesion molecule (NCAM), neurofilament proteins, synaptophysin, and chromogranin. NSP-A was observed in all cell lines to a different extent. However, no clear correlation was observed with the degree of neuronal differentiation as defined by other neuronal and neuroendocrine markers or morphology. NSP-B could not be detected. The induction of neuronal differentiation with nerve growth factor, dbcAMP, and retinoic acid in the rat pheochromocytoma cell line PC12 and the human teratocarcinoma cell line hNT2, respectively, induced the expression of NSP-A and NSP-C in these cell lines parallel to the induction of neurofilament protein expression. It is concluded that NSP-C expression, in particular, is strongly correlated with neuronal differentiation.

Data on the axonal projections of enteric neurones in the human intestine are still scarce. The present study aimed to identify the morphology and neurochemical coding of enteric neurones in the human small intestine, which are involved in the innervation of the mucosa. The lipophilic neuronal tracer DiI was applied to one mucosal villus of small intestinal resection specimens. The tissue was kept in organotypic culture and subsequently processed for immunohistochemistry. Neurones labelled from the mucosa were located in all ganglionated nerve networks, including the myenteric plexus. In all plexuses, at least ®ve neurochemical types of neurones could be observed, i.e. SOM-IR neurones, SP-IR neurones, SOM/SP-IR neurones, VIP-IR neurones and neurones lacking immunoreactivity for any of these markers. Most of the DiI-labelled neurones were multidendritic; a minority of neurones could be identi®ed as Dogiel type II cells, suggesting the existence of a subgroup of primary afferent neurones in the DiI-®lled cell population. The ratio of labelled multidendritic neurones (assumed to be secretomotor) to labelled Dogiel type II neurones (assumed to be primary afferent) in the myenteric plexus is higher in large mammals (pig and human) than in small mammals (guinea pig). This might point to the existence of a different topographical distribution of subsets of primary afferent neurones and/or topographically distinct intrinsic mucosal re¯ex circuits in large mammals, including humans.