Malignant mesothelioma

Photomicrograph of an epithelial malignant mesothelioma. The sheets of pleomorphic cells are epithelial in appearance, with eosinophilic cytoplasm and fairly well-defined cell borders. Cytoplasmic vacuoles (arrow) are present, which can lead to confusion with a signet ring type of adenocarcinoma. These vacuoles can be shown by electron microscopy to contain crystallized hyaluronic acid.