The fine structure of hyaline inclusions (pseudopsammoma bodies) in meningiomas.

"Hyaline inclusions" of a meningothelial meningioma were examined under the electron microscope. These eosinophilic, PAS-positive proteinaceous structures, which under the light microscope are seen in both extra- and intracellular location, were found by electron microscopy to consist of granular masses surrounded by cell membranes with microvilli. The "intracellular" bodies were also surrounded by similar microvilli in a space within the cells. Such spaces, variously known as intracellular ductules and "neolumen formation", have been previously described in mammary cancer, bronchiolar carcinoma and pleural mesotheliomas, among others, and have been, in the latter instances, regarded as signs of secretory differentiation. Thus, hyaline inclusions of meningiomas are different from truly intracellular hyaline bodies of neoplastic astrocytes (found by Rubinstein and Herman to be bodies within autophagic vacuoles) and may be regarded as a possible factor in ultrastructural differential diagnosis between meningiomas and gliomas.