The morphological classification of the different neuronal cell types is generally accepted and expanded by us; nevertheless, immunohistochemically and electrophysiologically the existence of clear-cut categories of enteric neurons is frequently questioned. The immunohistochemical results demonstrated in this study, however, provide the first direct link between a morphological type of enteric neuron and an immunohistochemical staining for a distinct peptide. Evidence demonstrates that calcitonin gene-related peptide occurs in only one morphologically defined type of neuron, viz. in type II neurons, and can therefore be regarded as a ‘marker peptide’ for type II neurons. Hence, the present immunohistochemical findings illustrate the validity of the morphological classification of the enteric neurons.

Calcitonin gene-related peptide (CGRP)-containing perikarya and axonal processes were localized by preembedding electron-microscopic immunocytochemistry in the porcine small intestine. Immunoreactive well-defined type II neurons were localized in the plexus myentericus, and plexus submucosus externus and internus. In some cases, they were found in direct contact to the basal lamina surrounding the ganlion, thus being in close apposition to the interstitial space. The perikarya are generally larger than the immunogative nerve cell bodies and have a typical smooth outline. The electron-microscopic features of the labeled nerve processes investigated provide evidence for their axonal nature. These ultrastructural observations confirm previous light-microscopic results which showed that CGRP-containing nerve cells in the porcine small intestine belong to the neuronal population of the type II cells, the processes of which display the ultrastructural features of axons. A large number of reactive varicosities show synaptic specializations on immunonegative nerve cell bodies, suggesting that at least part of the type II neurons have post-synaptic effects on CGRP-negative neurons.