Serotonin (5-HT) containing endocrine cells were visualized by an immunoperoxidase technique in gastric and duodenal mucosa of the dog. Numerous 5-HT immunoreactive cells were found throughout the gastric glands below the pits and the duodenal glands below the crypts. These cells were mostly located in the middle third of the gastric glands and in the lower portion of the duodenal glands, while relatively few were found in the neck portion of both glands. The 5-HT positive cells had various features such as pyramidal or bottle-like shapes. They were always situated within the basal lamina of the glands. Electron microscopic study revealed that the 5-HT immunoreactive structures seemed identical with the secretory granules of the enterochromaffin (EC) cells. The 5-HT positive granules showed polymorphic features having 200-300 nm in diameter.
The present study demonstrates that 5-HT containing EC cells can be detected with high sensitivity by the immunoperoxidase method, in which results are in agreement with previous reports obtained by various conventional techniques such as Falck-Hillarp's fluorescence histochemistry at a light microscopic level. Further evidence is given, for the first time by immunohistochemistry, showing specific subcellular localization of immunoreactive 5-HT in gastrointestinal endocrine ceells at an electron microscopic level.