ULTRASTRUCTURE OF THE HUMAN SPINAL ARACHNOID MATER AND DURA-MATER

Human spinal dura and arachnoid, obtained during neurosurgical operati ons, were studied by transmission electron microscopy. The ultrastruct ure of spinal meninges largely conformed to the morphology of the cran ial meninges, but some minor differences were detected. The dura was c omposed of an outermost loosely arranged fibroelastic layer, a middle basically fibrous portion and an innermost cellular layer (dural borde r cell layer). The dural border cell layer was characterised by multip le interdigitating cell processes, no extracellular collagen, signific ant extracellular spaces and few cell junctions. Paravascular vesicula ted nerve profiles were encountered within the fibroadipose epidural t issue. The arachnoid was composed of an butermost portion (arachnoid b arrier cell layer), presenting tightly packed cells, numerous tight ju nctions and no extracellular collagen. In view of its numerous tight j unctions, the arachnoid barrier cell layer is considered to represent an effective morphological and physiological meningeal barrier between the cerebrospinal fluid in the subarachnoid space and the blood circu lation in the dura. The arachnoid barrier layer was always characteris ed by a distinct continuous basal lamina on its inner surface towards the innermost collagenous portion of the arachnoid (arachnoid reticula r cell layer). The interweaving arachnoid trabecular cells within this layer possessed numerous mitochondria and were anchored to the inner surface of the arachnoid barrier cell layer by desmosomes. An addition al layer of flattened branching cells was demonstrated along the inner surface of the arachnoid reticular cell layer and assumed to be an 'a rachnoid border cell layer'. Morphological data suggest that the dura and arachnoid closely adhere at spinal levels in man without any natur ally occurring 'subdural space'. However, structurally, the dural bord er cell layer forms a weak cell layer at the dura-arachnoid continuum that is easily disrupted. The creation of an artifactual subdural spac e at spinal levels is discussed.