Substance P-immunoreactive neurons of the bovine forestomach mucosa: their presumptive role in a sensory mechanism.

Substance P-immunoreactive nerve fibers and cell bodies were examined in the forestomach mucosa of the calf and cow pretreated with colchicine, using thick (100 microns) floating sections. Intraepithelial nerve fibers were identified, appearing only rarely in the rumen and reticulum, and completely absent from the omasum. Nerve fibers were observed in the lamina propria of all the regions of the forestomach examined. A few thin nerve fibers were seen in the core of the ruminal papillae of the calf, whereas in the cow they appeared very coarse in nature. Flocculent and complicated nerve fiber networks were seen in the connective tissue of the reticular papillae. Mucosal nerve fibers formed a peculiar glomerulus-like architecture in the omasal papillae of the calf, while in the cow, the nerve fibers were largely restricted in distribution to the vicinity of the epithelium within the connective tissue pegs. Immunoreactive nerve cell bodies were found in the ruminal atrium, the dorsal sac and the ventral sac of the rumen of the calf and in the reticulum of both the calf and cow. Some of these neurons exhibited processes that appeared to course toward the papillae. In total, substance P-immunoreactive nerve fibers and cell bodies were more abundant in the calf than in the cow. These distributions demonstrate that the neural circuitry of the bovine forestomach contains substance P immunoreactivity in the mucosa as well as in the muscle layer, pointing to its possible importance in the regulation of the forestomach function. Substance P-immunoreactive nerve fibers were numerous in the reticular papillae of the calf and cow and in the omasal papillae of the calf. The positive fibers at these localities may act as mucosal receptors.