Microscopic anatomy of the brain-meningioma interface

We analyzed the relation between meningioma and the brain in 50 surgical cases. So-called capsule formation was seen in 20 meningiomas, of which 13 were categorized as thin and 7 as thick. In 21 meningiomas the arachnoid membrane was intact, and 10 meningiomas had no underlying arachnoid membrane. The other 19 tumors showed partial disruption of the arachnoid membrane. The degree of arachnoid disruption correlated with the tumor grade, perifocal edema, pial blood supply on angiography, and tumor size. The existence of brain invasion correlated with the tumor grade and partially with tumor size. In case of invasive tumor, GFAP-positive cells were found deep in the tumor, usually in contact with blood vessels. The axons in gliotic brain often showed degenerative changes such as ballooning or varicose swelling. Meningiomas were usually demarcated by a basement membrane that was collagen type 4 (Col4)-positive. However, atypical and anaplastic meningiomas usually lacked Col4 staining at the interface. In two benign meningiomas that looked like an invasive growth, Col4 staining was seen above the brain. A pia mater-like structure covered the tumor surface in both cases. We could not demonstrate a relation between the expression of matrix metalloproteinase (MMP)-2 or MMP-9 and arachnoid disruption or brain invasion.