Multicentric reticulohistiocytosis. Electron microscopic and ultracytochemical studies.

Lesions from a patient affected with multicentric reticulohistiocytosis (MR) were studied by ultracytochemical techniques to determine the relationship of PAS-positive materials and/or acid phosphatase (AP) activity to electron-dense granules found in the large mononucleated or multinucleated giant cells that are characteristic of this entity. The lead phosphate reaction products that mark the sites of AP activity were found in the electron-dense granules. The granules were also PAS positive by the periodic acid-thiosemicarbazide-silver proteinate method. These findings suggest that the PAS-positive and diastase-resistant materials in the large mononucleated or multinucleated giant cells of MR are not phagocytized materials but are pre-existing lysosomes.