A morphological and histochemical analysis of the neuroendocrine system of the gut in Acipenser transmontanus

Morphological, histochemical and immunohistochemical data are presented to demonstrate that the enteric nervous system of the sturgeon is in part composed and arranged differently from other fish. It is composed of neurons which distribute both to the tunica propria-submucosa and tunica muscularis. Nerve cell bodies are small and nerve terminals run in bundles which are both unmyelinated and myelinated. The presence of myelinated nerve fibres in the enteric nervous system of vertebrates is infrequent. Ganglionated plexuses are only found in relation to the musculature. In contrast with the other tracts of the gut, the musculature of the oesophagus is arranged into two orthogonal layers, and the inner layer is composed of striated muscle fibres. Enzymes related to the classical neurotransmitters acetylcholine and adrenaline and some possible accessory neuromediators (CGRP-, somatostatin-, ANP-, substance P-, NPY-like peptides, and nitric oxide) were identified histochemically and immunohistochemically in components of the enteric nervous system, especially those which innervate the oesophagus. The diffuse endocrine system was limited to a gastric cell type, which synthesized a somatostatin-like peptide. Some of these special features of the enteric nervous system may possibly be related to functional properties peculiar to the sturgeon gut, which also shows aspects of morphological organization that are different to those of other fish.