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.