Extremely intricate labyrinthine structures: an electron microscopic observation of peculiar invaginations of the plasma membrane found in secretory meningioma.

Large, extremely intricate labyrinthine structures were observed during an electron microscopic examination of a secretory meningioma that originated in the sphenoid ridge of a 73-year-old man. In many neoplastic cells, these labyrinthine structures formed round-shaped regions within the cytoplasm, and profiles indicated their internal structure consisted of numerous thin fragments of cytoplasm and intervening flocculent material. These labyrinthine structures did not exhibit apparent topographic association with pseudopsammoma bodies, and direct communication with the extracellular space by very thin channels was rarely observed. The structures resembled the "intracytoplasmic lumina" commonly seen in epithelial neoplasms; but they did not appear to be true "intracytoplasmic" closed structures but rather deep and elaborate invaginations of the plasma membrane into the cytoplasm. These structures are distinct from pseudopsammoma bodies and might represent another expression of the epithelial properties of neoplastic cells in this subtype of meningioma. Although their pathogenesis or significance remain unknown, these structures may significantly increase the surface area of neoplastic cells and facilitate the uptake of extracellular material.