Serotonin-immunoreactive cells in the lamina propria plexus of the appendix

The lamina propria of the human appendix possesses a recently defined ganglionated neural plexus with neurosecretory-type component cells. In order to elucidate the functional role of this plexus, immunohistochemical light and electron microscopic studies were performed on 21 appendixes. By light microscopy, 18 specimens showed within the lamina propria neuron-like and neurosecretory-type cells that stained positively with lead hematoxylin and were positive for serotonin. The number of these cells varied from specimen to specimen. No correlation between the number of serotonin-immunoreactive intraepithelial enterochromaffin cells and the number of plexus cells could be established. Electron microscopic examination revealed identical pleomorphic neurosecretory granules within enterochromaffin cells and plexus cells. The site of serotonin storage in these granules was revealed by electron immunocytochemical methods. It is suggested that, under physiologic conditions, the lamina propria plexus has a modulatory role between the epithelium and the deeper mural enteric nervous system mediated by serotonin neurotransmission. Uncontrolled release of serotonin by the plexus may initiate inflammation and elicit pain related to the appendix.