Morphological and histochemical analysis of the neuroendocrine system of the gut in Acipenser transmontanus.

The gut of adult sturgeon was examined. The oesophageal mucosa contained numerous caliciform cells, synthesizing both neutral and acidic glycoconjugates, the latter of the sialylated type. The deep tunica propria-submucosa contained lobules of multilocular adipose tissue, specially abundant during the cold season. The oesophageal tunica muscularis was made up of a large sheath of striated muscle fibres, arranged orthogonally to a thin, subserous smooth muscle layer. The siphon-shaped stomach showed a ciliated epithelium in cardiac and gastric proper gland zones, where tubular glands were present in the tunica propria. The columnar cells which composed the superficial epithelium and gastric pits were demonstrated to synthesize almost exclusively neutral glycoconjugates. Appendices pyloricae constituted a glandular body equipped with intestinal mucosa. The intestinal mucosa was organized in folds, containing numerous caliciform cells which synthesized neutral or acidic glycoconjugates, the latter either of the sialylated and sulphated type. The sulphoglycoconjugates were more abundant in the caliciform cells of the distal intestinal tracts. The tunica propria-submucosa of the spiral valve (medium intestine) contained lymphatic tissue and large lymphatic follicles. A muscularis mucosae was present only in the rectum, where in addition a peculiar granular cell type was present in the superficial tunica propria-submucosa, possibly related to defensive properties. The subserous connective tissue contained pancreatic lobules all along the stomach and intestine. The enteric nervous system showed some special aspects, the most intriguing of which was the presence of large, longitudinally oriented nerve bundles in the t. propria-submucosa of oesophagus and cardiac stomach. The nerve bundles contained, near unmyelinated nerves, some myelinated nerves, as well as neuronal bodies. Both these aspects are exceptional in vertebrates and obscure in their significance. The structural and histochemical aspects we here describe are in part different from those described for other fish. Some of these special features are possibly related with special functional roles, others require a deeper insight and different approaches to clarify them functionally.