A case of malignant mesothelioma containing fibrous long-spacing fibers in the sarcomatous stroma.

Fibrous long-spacing fibers (FLS) of a 90- to 120-nm period found in malignant mesothelioma of a 73-year-old woman were described. The FLS were observed in the reticular stroma with a fibroblastic variety of tumor cells, and they appeared to blend with bundles of thin collagen fibers and a moderate amount of ruthenium red (RR)-positive materials. No FLS were recognizable in the myxomatous stroma with abundant RR-positive materials or in the collagenous stroma packed with thick collagen fibers. Therefore, the presence of both thin collagen fibers and tumor cells of fibroblastic differentiation is presumed to be responsible for the formation of the FLS in the particular tumor.