Benign papillary mesothelioma of peritoneum: a histological, histochemical and ultrastructural study of six cases.

Six cases of localized benign papillary mesothelioma of peritoneum were retrieved from laboratory files over a 20-year period. One presented because of pain following torsion and the remaining five were incidental findings at operation. Light microscopy showed a delicate papillary stroma covered by a single layer of mesothelial cells. The mesothelial cells often showed punctate periodic acid-Schiff staining of cytoplasm. The brush border and stroma showed alcian blue positive staining which was abrogated by prior incubation with hyaluronidase. Electron microscopy revealed typical features of mesothelial cells with long microvilli, rough endoplasmic reticulum in relation to mitochondria, bundles of microfilaments, and tight junctions. Between cells there was a series of intercommunicating channels containing occasional lymphocytes and debris. These features are compared with nodular mesothelial hyperplasia, malignant mesothelioma and adenomatoid tumours of the genital tract. The necessity to distinguish benign papillary mesothelioma from metastatic carcinoma is stressed and relies on the recognition of their mesothelial origin.