Immunohistochemical and ultrastructural study of the enteric nervous system of earthworm, Lumbricus terrestris L.

Light and electronmicroscopic data reveal the presence of a well developed nerve plexus in the gut of the earthworm. The plexus contains subepithelial solitary nerve cells and fibers and an extensive neuropil among the muscle cells. There are two types of nerve cells in the enteric plexus. The first type contains mainly dense-core vesicles, and exhibits glyoxylic-acid induced fluorescence. Since none of these cells showed serotonin immunoreactivity, they are probably noradrenergic or dopaminergic. The second type contains large dense granules, suggesting that these cells are peptidergic (neurosecretory). A part of these cells are substance P immunoreactive, however no NPY, CGRP, or proctolin immunopositive cells were found. Ultrastructurally seven types of nerve fibers can be distinguished in the neuropil. Their distribution shows great variability within parts of the enteric canal. The observation that only two types of nerve cells are located within the gut makes it probable that some of the axons are extrinsic. According to immunohistological studies they may come from the stomatogastric system or from the segmental nerves. This is further supported by the fact that there is a well-developed subepithelial serotoninergic plexus in the fore-gut. Two types of neuromuscular junctions can be visualized in the muscular layer. The first type, representing a phylogenetically earlier form, exhibits wide junctional gap and pre- or postjunctional membrane thickening. The second type is the close contact. There are significantly more junctions observed in the fore-gut than in other parts of the gut.