Cytokeratin expression in small intestinal and appendiceal carcinoids. A basis for classification

For a study of histogenesis of intestinal carcinoids a collection of 5 classical small intestinal carcinoids, 6 appendiceal carcinoids and 9 pheochromocytomas, were evaluated. The tumors were identified by routine morphology, silver staining and chromogranin immunocytochemistry and were then examined with regard to the expression of intermediate filaments of cytokeratin type. Eight different antisera identifying individual or combinations of cytokeratins were employed. All classical small intestinal carcinoids displayed cytokeratin immunoreactivity and an almost identical cytokeratin reaction was observed in the normal enterocytes of the small intestinal mucosa. Of the individual cytokeratin types, number 18 was most heavily expressed. The appendiceal carcinoids, like the pheochromocytomas, almost totally lacked a cytokeratin staining despite a positive reaction in the mucosa of the appendix. This, in agreement with some previous studies, indicates that the small intestinal carcinoids are histogenetically related to the epithelial cells of the intestinal mucosa, while the appendiceal carcinoids have a different histogenesis and are more like pheochromocytomas. The appendiceal carcinoid may represent a distinct type of intestinal paraganglioma. This offers one explanation for the different biological behavior of appendiceal carcinoids in comparison with the other intestinal carcinoids.