Differential distribution of serotonin and tryptophan hydroxylase in the human gastrointestinal tract

The distribution of tryptophan hydroxylase, the rate-limiting enzyme in the biosynthesis of serotonin, was investigated immunohistochemically in vario us organs of the gastrointestinal tract and compared with that of neuroendo crine markers. While immunoreactivity for serotonin and chromogranin A was restricted to enterochromaffin cells, positive staining for tryptophan hydr oxylase was detected in normal enterocytes lining the epithelium of the sma ll intestine. Tryptophan hydroxylase was localized in the supranuclear cyto plasm of absorptive cells, and was absent from the terminal web, The entero cytes of the exfoliation zone at the tips of the villi demonstrated a stron g immunoreactivity similar to those at the slope of the villi. Mucus-contai ning Goblet cells, Paneth cells and stromal cells of the lamina propria rem ained unlabelled. The duodenal glands of Brunner revealed only sporadically a weak immunostaining for tryptophan hydroxylase, The monooxygenase was al so detected in numerous secretory tubules of the pyloric mucosa, where the proportion of positive cells decreased progressively from the crypts toward s the upper parts of the gastric glands. No significant immunoreactivity wa s demonstrated in colon, adrenal cortex, liver, pancreas, and mesenteric ly mph nodes, The demonstration of tryptophan hydroxylase in normal enterocyte s suggested that epithelial cells of the small intestine are able to synthe size 5-hydroxytryptophan.