Identification of the APUD endocrine cells of rat fundic mucosa by means of combined amine fluorescence and electron microscopy.

Rat fundic mucosa contains numerous APUD endocrine cells that can take up 5-hydroxytryptophan (5-HTP) and decarboxylate it to 5-hydroxytryptamine (serotonin), detectable by its formaldehyde-induced yellow fluorescence. To identify these cells by electron microscopy, pieces of rat gastric mucosa were incubated with DL-5-HTP. Some specimens were fixed in 4% formaldehyde-0.5% glutaraldehyde, while others were frozen, freeze-dried, and exposed to paraformaldehyde vapors. Thick sections of the Epon-embedded specimens were examined and photographed by fluorescence microscopy, and fluorescing and nonfluorescing cells were identified by electron microscopy in serial thin sections. Control specimens, not incubated with 5-HTP, revealed fluorescence of only the mast cells and EC cells, which were abundant in the pylorus, but rare in fundic mucosa. Specimens incubated with 5-HTP also exhibited numerous yellow fluorescent endocrine cells in fundic mucosa, which cells were found to be the ECL cells; the A-like cells did not fluoresce. In pyloric mucosa, many G and D (D1) cells also exhibited weak or moderate fluorescence after 5-HTP incubation. Thus, this study supports the contention of previous radioautographic studies that the ECL, but not the A-like, cells are the APUD endocrine cells of rat fundic mucosa, that G and D cells also possess some APUD activity, and that EC cells represent the enterochromaffin cells, which normally synthesize and store demonstrable quantities of serotonin.