The origin of inner membranes in chronic subdural hematomas.

Electron-microscopic findings of inner membranes of chronic subdural hematomas showed multilayered tiers of flattened cells. The basic characteristics of these cells were similar to dural border cells in the human dura-arachnoid interface layer. The cells covering the hematoma surface had indented nuclei with a prominent nucleolus and were abundant in enlarged rough ER, glycogen granules, lipid droplets, and caveolae. The cells in the intermediate layer had thin cytoplasmic extensions containing tonofilaments, which were oriented almost parallel to the long axis of inner membranes. The cells facing the arachnoid surface sometimes showed disintegration of cellular organelles and dissolution of nuclear chromatin. Between these cells and the tiers was an increased amount of extracellular substance, such as collagen fibrils, elastins, and finely granular material, which were often intermingled with blood pigments or fibrins, especially toward the arachnoid surface. In two of the ten cases studied, there was a syncytial mass of arachnoid cells which reinforced the arachnoid surface of inner membranes. Conceivably, a primary extravasation of blood within the dura-arachnoid interface layer may cleave a few tiers of dural border cells, which envelope the inner surface of the hematoma, proliferate, and later on form inner membranes.