[Origin of the capsule of a chronic subdural hematoma--an electron microscopy study].

To understand the early stage of subdural neomembrane (SN) formation, we examined 23 outer SNs and two inner SNs of chronic subdural hematomas in surgical specimens. Ultrastructurally, the cellular component of the outer SN was fibroblasts, myofibroblasts, smooth-muscle cells (in four cases), blood vessels and blood-born cells. Dural border cells were observed only in limited areas. Namely, the outer SN is a simple granulation tissue which originates from multiple sources of mesenchymal cells in dura mater, and dural border cells are only one of constituent sources for SN formation. The inner SN was simply composed of slender spindle cells which were mostly fibroblasts and some were recognized as dural border cells. Our results showed that the subdural hematoma is actually an "intra" dural hematoma which is formed within the split dural border cell layer. The dura mater with rich vascular supply forms thick granulation tissue, that is outer SN and a few dural border cells on the underlying arachnoid membrane form thin fibrous inner SN.