We have previously demonstrated that gastric and intestinal endocrine cell (End-cell) marker expression is important for assessment of the histogenesis of endocrine cell tumors. However, the End-cell phenotypes of carcinoid tumors in the rectum remain largely unclear. We therefore examined marker expression of rectal carcinoid tumors. We evaluated 20 rectal carcinoid tumors (as well as 8 from the stomach for comparison) phenotypically, using gastrin, gastric inhibitory polypeptide (GIP) and glucagons-like peptide-1 (GLP-1) as End-cell markers. Rectal carcinoid tumors were divided into 3 endocrine-gastric (e-G), 16 endocrine-gastric-and-intestinal mixed (e-GI), 1 endocrine-intestinal (e-I), and 0 endocrine-null (e-N) types, thus 19 (e-G+ e-GI types, 95%) had gastric phenotypic expression, while 17 (e-GI+ e-I types, 85%) harbored intestinal elements. Stomach carcinoid tumors were classified as 6 e-G and 2 e-N types, respectively. In conclusion, most rectal carcinoid tumors exhibited the e-GI type, suggesting the importance of gastric End-cell marker expression for histogenesis of the rectal carcinoid tumors. Further studies of pathological and biological analyses are needed to clarify the histogenesis of the carcinoid tumors.
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