Adenomatous differentiation of typical carcinoid tumors was examined by histochemical and ultrastructural studies on 33 cases of typical carcinoids of the hindgut. Of these 33 carcinoids, 12 (36%) showed a tubular and/or acinar pattern with production of mucus as a minor component. A few tumor cells with mucous globules were scattered among tubules in these carcinoids. Ultrastructurally, microvilli were seen along the luminal surface of tubules, with both numerous mucous globules in the supranuclear area and endocrine granules in basal sites of the same cells. These results indicate that even in typical carcinoid tumors, certain tumor cells have the features of goblet cells or the open type of endocrine cells. The findings of both mucous globules and endocrine granules in the same single cells indicate a close relationship between endocrine cells and mucus‐producing and/or tubule‐forming cells. It is concluded that pluripoten‐tial ceils producing carcinoid tumors might be able to differentiate into three types of tumor cells: (i) endocrine cells without microvilli or mucus production
(ii) cells with microvilli and no mucus product
and (iii) mucus‐containing cells with microvilli. Copyright © 1994, Wiley Blackwell. All rights reserved