Ultrastructural comparison of arachnoid villi and meningiomas in man

An ultrastructural comparison of arachnoid villi and meningiomas was made in man. Human arachnoid villi basically consisted of four portions: fibrous capsule; arachnoid cell layer; cap cell cluster; and central core. Arachnoid cells had many characteristic structures in common with meningioma cells: plasmalemmal interdigitations; desmosomes; hemidesmosome-like junctions; and cytoplasmic filaments. Both arachnoid villi and meningiomas were characterized by the numerous extracellular cisterns which appeared to form outflow channels of cerebrospinal or serum fluid. Differences between arachnoid villi and meningiomas pertained to cytoplasmic components reflecting the states of permeability of endothelial cells, junctional complexes, and matrix substances of psammoma bodies. Endothelial cells investing the fibrous capsule contained a number of micropinocytotic vesicles and intracytoplasmic vacuoles, but showed no fenestrations. In contrast, meningiomas not only contained numerous micropinocytotic vesicles and intracytoplasmic vacuoles but also exhibited numerous fenestrations. Furthermore, there were a larger number of junctional complexes between arachnoid cells than meningioma cells. Arachnoid villi contained abundant matrix granules while meningiomas contained only a few matrix granules but a larger number of matrix minerals and vesicles, as the presumptive precursors of psammoma bodies.