Ultrastructural evidence for an innervation of epithelial enterochromaffine cells in the guinea pig duodenum

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Abstract
The innervation of the duodenal enterochromaffin cells (E.C.) of the guinea-pig was studied with EM. Pretreatment with 5-hydroxy-dopamine was performed to visualize catecholaminergic (CA) nervous elements. Near the basement membrane of all examined E.C. in the crypts, bundles of unmyelinated nerve processes were observed, only partly ensheathed in a Schwann cell cover. At least 4 types of processes could be observed: boutons containing only small clear vesicles, probably cholinergic fibers; boutons with small clear vesicles, and in addition large (> 200 nm) granules with a dense matrix (P-type-fibers); boutons with small electron-dense vesicles, probably CA-fibers; and processes with few vesicles but having the appearance of dendrites. No typical synaptic arrangements were observed, but the minimal distance between the E.C. and the nerve bundles was 150 to 250 nm, well within the functional limits of the autonomic gap. Epithelial E.C. may be influenced by several types of nervous elements, including CA-fibers.