Rectal carcinoids as tumors of the hindgut endocrine cells: a morphological and immunohistochemical analysis.

The rectal mucosa is richly endowed with a constellation of amine and polypeptide hormone-producing endocrine cell types which may be identified by silver staining and immunohistochemical methods. In order to study the relationships of rectal carcinoid tumors to the normal hindgut endocrine cells, rectal carcinoids and normal rectal mucosa were compared for the presence of argentaffinity and argyrophilia and for the distribution of a battery of polypeptide hormones. Normal rectal mucosa contained frequent cells which stained for bovine pancreatic polypeptide (PP), human PP, and glucagon-like immunoreactivity (GLI. Somatostatin (SRIF) was present in a smaller proportion of rectal endocrine cells. Both argentaffin and argyrophil cells were encountered frequently in normal rectal mucosa. In the series of 13 rectal carcinoids examined, two cases were focally argentaffin-positive, while eight tumors revealed varying degrees of argyrophilia. Eight tumors contained immunoreactive bovine PP, and four of these tumors which were tested for human PP were also positively stained. SRIF was present in five cases, while GLI was identified in two tumors. Four of the tumors were multihormonal. Rectal carcinoids have a rich polypeptide hormone content which parallels that of the normal rectal mucosa. The distinctive hormonal profile and silver staining properties may prove to be of value as specific markers for carcinoid tumors of rectal or hindgut origin.