Mucins and neuromodulators involved in mucus secretion in the alimentary canal of Siluriformes

The morphological diversity of the fish alimentary canal depends on many factors, mainly the diet and phylogeny. We studied the relative densities of the diverse histochemical mucous cell populations in the epithelium of the alimentary canal of the black bullhead, Ameiurus melas, and the wels catfish, Silurus glanis. The classical histochemical protocols indicated a comparable pattern of mucous cells between the two species, with predominant mixed mucins in the esophagus, neutral mucins in the stomach, and acidic mucins in the intestine. Moreover, mucous cell types were characterized by the application of 8 lectins which showed the presence of specific carbohydrate terminal residues on mucins of the alimentary canal. In addition, an immunohistochemical protocol was used employing 5 antibodies against molecules involved in mucus secretion, namely choline acetyltransferase, serotonin, galanin, and leu- and met-enkephalin. These antibodies revealed a relatively high abundance of immunoreactive endocrine cells, as well as immunoreactive nerve cells in the myenteric plexus, likely having a role in mucus discharge. The significant differences observed in the sugar residues between the two species could be related to their specific feeding ecology, carnivorous for S. glanis and omnivorous for A. melas. The presence of plant polysaccharides in the black bullhead diet needs more complex mucosal components than the wels catfish diet which involves reduced fermentative processes.