A unique population of uranaffin-positive intrinsic nerve endings in the small intestine.

A positive uranaffin reaction was observed in the small (40-60 nm) diameter vesicles of some intestinal axons. There was no change in the number of reactive axons or the intensity of reaction after reserpine (5 mg/kg) or after interruption of axons reaching the intestine through the mesentery. The axons were found in the myenteric, submucous and deep muscular plexuses and in the circular muscle. Some uranaffin-positive axons formed synapses with neurons of the myenteric and submucous plexuses. It is concluded that these axons are not noradrenergic. The axons must represent one of the several nerve types which are known to be intrinsic to the intestine, but are as yet unidentified at an ultrastructural level. If, as has been postulated, the reaction localizes amine storage vesicles, the uranaffin-positive axons are probably the intrinsic amine-handling axons previously demonstrated histochemically.