On the question of a subdural space.

The structure of the meninges, with particular attention to the architecture of the inner portions of the dura mater and the arachnoid mater, has been reviewed in reference to the probable existence of a "subdural" space. The dura is composed of fibroblasts and large amounts of extracellular collagen. The innermost part of the dura is formed by the dural border cell layer. This layer is characterized by flattened cells with sinuous processes, extracellular spaces containing an amorphous material, and the presence of junctions between its cells. The dural border cell layer is continuous with the inner (meningeal) portions of the dura and may be attached to the underlying arachnoid by an occasional cell junction. The arachnoid consists of an outer part, the arachnoid barrier cell layer, and an inner portion, the arachnoid trabeculae which bridge the subarachnoid space. Arachnoid barrier cells are electron-lucent, closely apposed to each other, and joined by many cell junctions; in this layer there is little extracellular space and essentially no intercellular material. Arachnoid trabecular cells cross the subarachnoid space in a random manner, have extracellular collagen associated with their flattened processes, and form structures of variable shapes and sizes. There is no evidence of an intervening space between the arachnoid barrier cell layer and the dural border cell layer that would correlate with what has been called the subdural space. When a tissue space is created in this general area of the meninges it is the result of tissue damage and represents, in most instances, a cleaving open of the dural border cell layer. In this situation, extracellular spaces in the dural border cell layer are enlarged, cell junctions are separated, and it is probable that cell membranes are damaged. A survey of reports describing the morphology of the inner and outer capsule of so-called subdural hematomas in humans reveals that dural border cells are found in both parts of the capsule. Also, experimental infusion of blood into this portion of the meninges in animals frequently dissects open the dural border cell layer. These data support the view that what has been called a subdural hematoma is most frequently a lesion found within the layer formed by dural border cells. It is suggested that the so-called subdural space is not a "potential" space since the creation of a cleft in this area of the meninges is the result of tissue damage. In this respect it shares no similarities with legitimate potential spaces (i.e., serous cavities) found at other locations in the body.(ABSTRACT TRUNCATED AT 400 WORDS)