The subdural space interpreted as a cellular layer of meninges.

The subdural region within the cranial meninges is examined in guinea pigs by electron microscopy. The fine structures of the arachnoid membrane and dura are described separately in specimens that have been isolated from each other during removal from the cranial cavity. In addition, the fine structure of the intact dura-arachnoid is described, where the subdural space would be present in an undisrupted state. Lastly, the inner surface of the dura and the outer surface of the arachnoid membrane are examined at the point of separation between the two specimens where the dura is reflected from the arachnoid by experimental dissection. From these observations morphologic criteria are established for identifying the constituents and boundaries of the subdural space and for explaining mechanisms in the histogenetic process of "opening" or enlarging this space. The morphologic identity of the classic subdural space is reinterpreted in light of the findings. The subdural space, traditionally described as a fluid-filled potential cavity existing in an extracellular compartment, is not apparent in the guinea pig. Instead, fragile cells designated as light cells occupy the compartment between the dura and arachnoid, with very little extracellular space available. Experimental opening of the subdural space occurs, significantly, along pathways extending by fracture through the cytoplasm and intercellular separation of these light cells rather than by enlargement of a preexisting mesothelial-lined intercellular space between these cells and the true arachnoid cells. Cytoplasmic fine structure of light cells suggests a close kinship with cells in the meningeal layer of the dura. The functional significance of the light cells and their possible role in subdural hematomas is discussed.