On Arachnoid Villi and Meningiomas: Functional Implication of Ultrastructure, Cell Adhesion Mechanisms, and Extracellular Matrix Composition.

Arachnoid villi or granulations are small projections of the arachnoid barrier layer into the venous sinus and its major tributaries. They are closely related to the absorption of cerebrospinal fluid, and are widely accepted to be the origin of human meningiomas. Arachnoid villi and meningiomas show a number of similarities in ultrastructure, cell adhesion mechanisms, and extracellular matrix composition. Ultrastructurally, both arachnoid and meningioma cells are characterized by interdigitations connected with junctional complexes, and extracellular cisterns related to the fluid transport. Extracellular cisterns and the intercellular space reveal abundant membrane-derived multilamellar phospholipids when a conventional ultrastructural fixative supplemented with tannic acid is used. Both arachnoid and meningioma cells are connected by Ca2+-dependent adhesion molecules: epithelial-cadherins which are concentrated at the adherens junctions. Membrane-cytoskeleton interactions by means of merlin and a-catenin molecules are thought to be crucial in signal transduction resulting in contact inhibition of cell growth in normal arachnoid cells. Impairment of these molecules might be related to meningioma-genesis. Glutathione-independent prostaglandin D2 synthase [EC 5.3.99.2] responsible for the biosynthesis of prostaglandin D2 in the central nervous system is also consistently expressed in human arachnoid villi and meningiomas. The multilamellar phospholipids are conceivably related to this arachidonate metabolism.