Paneth cell-rich flat adenoma of the rectum: report of a case.

A patient having familiar adenomatosis polyposis and an ileo-rectal anastomosis developed a flat mucosal lesion in the rectum. A punch biopsy revealed a villous adenoma with high-grade dysplasia. The subsequent surgical specimen indicated that the flat villous adenoma was rich in Paneth cells. Special stains included lysozyme muramidase (to visualize Paneth cells), MIB1 proliferation monoclonal antibody and single and multilabel immunohistochemistry for Paneth cells. Other methods included transmission electron microscopy and quantification with an image quantifier (Program Optilab 2.1) of lysozyme-stained Paneth cells. The subjective evaluation of hematoxylin-eosin-stained preparations demonstrated that the Paneth cells were mainly located in the lower half of the villi. Sections labeled with a specific stain (lysozyme muramidase) revealed more Paneth cells in the villi and electron microscopy showed even more in lysozyme-negative areas. Obviously some migrating dysplastic Paneth cells had retained their characteristic granules on their way towards the tip of the villi. Quantitative studies indicated that the lysozyme muramidase-positive material accounted for 41% of the adenomatous tissue. MIB1 revealed intense cell proliferation at the base of the adenoma and in the entire slopes of the villi. Despite the wide distribution of Paneth cells in intestinal metaplasia of the stomach, in the normal small intestine and in the large bowel with chronic inflammatory diseases, it is surprising that tumors arising in Paneth cells are extremely rare. The causes of the apparent natural resistance of Paneth cells to tumor development deserve to be investigated. This is the first case of Paneth cell-rich flat adenoma of the rectum in the literature.