Histochemical and immunohistochemical characterization of foamy histiocytes (muciphages and xanthelasma) of the rectum.

Despite being relatively common in the rectum, foamy histiocytes have received scant attention as to the antecedent lesion that causes them to form or their histologic characterization on the types of muco-substances they accumulate. One-hundred consecutive tissue sections of the rectum from an equal number of patients were reviewed for the presence of foamy histiocytes, evaluated for their associated histologic features, and examined histochemically for five types of mucin. Immunohistochemical and electron microscopic studies were performed. Forty (40%) of the rectal biopsy tissues contained foamy histiocytes. Patients presented with diarrhea, hematochezia, intestinal habit change, constipation, hemorrhoids, and abdominal pain. Endoscopically, 19 patients were thought to have rectal nodules or polyps. Histologically, 25 of the patients had regenerative changes in the adjacent mucosa and 14 had hyperplastic changes. In 36 patients (90%), the foamy histiocytes were located superficially in the lamina propria. Associated changes indicated that they are found in areas that are subject to an injury that is in a healing phase. These changes included mild fibrosis and chronic inflammation of lamina propria with mild architectural distortion. Thirty-five (88%) cases showed staining for D-PAS, Alcian blue stain pH 2.5, and the cocktail Alcian blue stain/PAS. Mucicarmine was positive in 25 (63%) cases. The Alcian blue stain pH 1.0 was positive in 19 (59%) of 32 cases. Ultrastructural studies showed electron-dense globules. Two cases were histologically identical to the other 38 but they did not stain for any mucin. Ultrastructural features disclosed clear vacuoles and thus represent a xanthelasma of the rectum. The foamy cells in all cases were confirmed to be histiocytes by immunohistochemistry and electron microscopy. Although muciphages and xanthelasma of the rectum may mimic polyps endoscopically, they are not related to any specific symptom or clinical finding, despite the fact that they probably represent remnants of a previous injury. Muciphages contain neutral, weakly acidic or strongly acidic mucin. The main type of acidic mucin is sialomucin with a smaller component of sulfated mucin.