Crypt cell carcinoma of the appendix (so-called adenocarcinoid tumor).

A series of 16 carcinoid tumors of the appendix, including four examples of so-called adenocarcinoid tumors, has been studied immunohistochemically for the presence of lysozyme, secretory component (SC) and IgA, within tumor cells. Stains for mucin, Paneth cells, and APUD cells were also performed. Of the conventional carcinoid tumors, eight showed focal tubule formation with production of PAS-positive material. In contrast to conventional carcinoids, including those showing tubule formation, adenocarcinoids contained few APUD cells and showed positive staining of tumor cells for lysozyme, SC, and IgA. Paneth cells, staining positively for lysozyme, were present in two cases and mucin-containing Paneth cells were observed. A parallel study of normal small intestinal mucosa demonstrated a population of lysozyme containing goblet cells within the crypts. It is suggested that so-called adenocarcinoid tumors are not derived from APUD cells but from lysozyme-producing cells of the type normally present in small intestinal crypts.