www.neurorgs.com - Unidad de Neurocirugía RGS The Origin of the Spinal Subdural Space: Ultrastructure Findings Miguel Angel Reina, MD*, Oscar De Leon Casasola, MD†, Andre´s Lo´pez, MD*, Jose´Antonio De Andre´s, MD‡, Miguel Mora, MD§, and Agustı´n Ferna´ndez, MD࿣ *Department of Anesthesiology and Critical Care, Hospital General de Mo´stoles, Hospital de Madrid Monteprı´ncipe, Spain; †Department of Anesthesiology and Critical Care Medicine, Roswell Park Cancer Institute, Buffalo, New York; ‡Department of Anesthesiology and Critical Care, Hospital General Universitario, Valencia, Spain; §Department of Urology, Hospital de Mo´stoles, Madrid, Spain; ࿣Electron Microscopy Center, Complutense University, Madrid, Spain Previous studies of samples from cranial meninges There was no subdural space in those specimens where have created doubts about the existence of a virtual sub- the dura mater was macroscopically in continuity with dural space. We examined the ultrastructure of spinal the arachnoid trabecules. In the specimens where the meninges from three human cadavers immediately af- dura mater was separated from the arachnoid, we ter death to see whether there is a virtual subdural space found fissures in between the neurothelial cells that ex- at this level. The arachnoid mater had two portions: a tended throughout the interface. We hypothesize that compact laminar portion covering the dural sac internal the subdural space would have its origin within the surface and a trabecular portion extending like a spider dura-arachnoid interface when the neurothelial cells web around the pia mater. There was a cellular interface break up, creating in this way a real subdural space. between the laminar arachnoid and the internal layer of the dura that we called the dura-arachnoid interface. (Anesth Analg 2002;94:991–5) he subdural space is described by textbooks as “a electron microscopy from samples of spinal meninges potential cavity between the dura and arachnoid to evaluate the presence or absence of a subdural T mater” (1–3); it has been visualized by contrast space. injection, and epidural catheters have been introduced using radiological techniques (4–8). However, when Blomberg (9) attempted to evaluate the subdural space by endoscopic methods in cadavers, he could not vi- Methods sualize it in all cases. The dural sac and its neural content at the lumbar During the last decades, different authors (10–16) level were removed from three donors immediately have studied the ultrastructure of the cranial dura- after their death. Approval from the research ethics arachnoid interface in animal and human meninges committee and family consent for the donation of the and have doubted the existence of a subdural space. organs and the procedures included in this research Vandenabeele et al. (17) studied samples of the spinal was obtained. The subjects were 48, 55, and 60 yr of meninges, and they could not find it. In all these age and had been admitted to the intensive care unit studies, the dura-arachnoid interface has been identi- because of cerebral hemorrhage and subsequently di- fied by different names: the medial border of the spi- agnosed with brain death. The laminectomy and dis- nal dura mater (10), dural border cell layer (18), sub- section were performed with the help of an orthopedic dural mesothelium (13), or subdural compartment surgeon. The dural sac was removed in a block from (14). We observed the ultrastructure of the dura- T11–L5 section. Extracted specimens containing the arachnoid interface under transmission and scanning dural sac were studied without further manipulation by scanning and transmission electron microscopy. Supported, in part, by the Fund of Investigation of the Ministry of After subsequent dissection, they were also studied by Health of Spain, Project 98/0628. surgical optical microscopy. Care was exercised to Accepted for publication November 27, 2001. observe specimens where the dura mater was in con- Address correspondence and reprint requests to Miguel Angel Reina Perticone, MD, Valmojado, 95 1st B, 28047, Madrid, Spain. tinuity with the trabeculated portion of the arachnoid Address e-mail to [email protected]. as well as specimens where the laminar arachnoid was ©2002 by the International Anesthesia Research Society 0003-2999/02 Anesth Analg 2002;94:991–5 991 www.neurorgs.com - Unidad de Neurocirugía RGS 992 REGIONAL ANESTHESIA REINA ET AL. ANESTH ANALG THE ORIGIN OF THE SPINAL SUBDURAL SPACE 2002;94:991–5 macroscopically separated from the dura mater as a The subdural space, as classically described, was result of the dissection. not present (TEM) in the specimens where the dural sac had its entire thickness and the dura mater was Preparation of Samples macroscopically in continuity with the arachnoid tra- beculae. The dura-arachnoid interface was occupied Transmission Electron Microscope. The specimens by neurothelial cells, and there was not a real space were fixed for4hinasolution of glutaraldehyde 2.5% between the arachnoid and dura mater membranes and a buffer phosphate solution to a pH value of 7.2–7.3. They were later fixated with a solution of 1% (TEM, SEM; Fig. 2). These cells were arranged concen- trically below the dura mater, and their thickness var- osmium tetroxide for 1 h. The specimens were dehy- ␮ drated with acetone and were soaked in resin epoxy ied between 5 and 7 m, distributed in 4–8 parallel Epon 812. Control group slides were dyed with Rich- cellular planes (TEM). The length of each neurothelial cell was longer than 100 ␮m, and their width varied ardson’s methylene blue dye. Ultra thin slides of 70 ␮ nm of thickness were made with an ultramicrotome between 0.5 and 1 m (TEM, SEM; Fig. 3). They were and treated with 2% acetate of uranilo solution and oriented in different directions, and their morphology with Reynolds lead citrate solution. The specimens presented numerous ramifications (SEM; Fig. 3). The were observed under a Zeitz transmission electron union of elongated and flat cells formed each plane microscope (EM 902; Carl Zeitz, Oberkochen, with multiple intercellular “digitlike” cytoplasmic Germany). prolongations (TEM). Depending on the cells studied, Scanning Electron Microscope. The samples were an intercellular space or cells joined by scarce desmo- fixed by immersion for4hin2.5% glutaraldehyde and somes or other specialized bridging were found. phosphate solution buffered to a pH value of 7.2–7.3. When the intercellular space was present, its thickness They were later dehydrated in acetone and pressur- varied. Sometimes, this space resembled lacunar ized under CO2 to reach the critical point. A carbon spaces occupied by amorphous material where the layer was then deposited on the samples to a thickness collagen or elastic fibers were almost absent (TEM; of Ͻ200 Armstrong and covered with a gold micro- Fig. 2). film. The samples were studied with a JEOL JSM 6400 The lack of fibers between these cells makes them scanning electron microscope (JEOL, Tokyo, Japan). different from adjacent cellular planes in the dura mater and the arachnoid membrane (TEM). The neu- rothelial cells presented a nucleus with disperse chro- Results matin, few mitochondria, a poorly developed endo- plasmic reticulum, and pinocytotic vesicles (TEM). In Observations During the Dissection. We observed a translucent membrane that detached from the internal those areas with fissures, the tearing extended mainly coating of the dural sac when light pressure or light through the amorphous material. In larger planes, a friction was applied on its internal surface. This mem- fissure was generated and expanded towards the lat- brane wrapped around all the nerve roots at their exit eral zones where a significant amount of amorphous from the medullary cone and the spinal cord, joining material was usually present (TEM). Once the epicen- the internal surface of the dura mater only in the areas ter of the fissure is established, it enlarges towards where the nerve roots come out from the subarach- areas of low mechanical resistance where the amor- noid space to cross the dural sleeves and in some phous substance is located between the neurothelial isolated randomly located points within the dural sac cells. There were lacunar zones with large amounts of (Fig. 1). As a result, this membrane formed a cylinder amorphous substance. In these zones, the coherence with lateral short sleeves and edges that joined the and mechanical resistance appeared to be less because internal surface of the dural sleeves. the fissure developed easily. When the tips of a new Electron Microscopy. The results of the findings ob- fissure hit a more resistant zone, like joints between tained by transmission electron microscopy (TEM) or neurothelial cells, its advancement causes one of these scanning electron microscopy (SEM), referring to the cells to break or, if the generated forces are too weak, same structure, will be described by indicating the the fissure expands in another direction (SEM). This method in parentheses. The arachnoid membrane was explains the presence of folded cellular fragments at seen as a compact laminar portion covering the dural the surface of the generated subdural fissure (SEM). sac in its inner surface and a trabeculated portion that In those specimens where the dura mater was mac- extended like a spider web to the pia mater of the roscopically separated from the arachnoid, we could spinal cord and the medullary roots (SEM). Between observe two laminae with a thickness of 300 and 40 the laminar arachnoid portion and the most internal ␮m, respectively (SEM; Fig. 4). Both laminae were layer of the dura mater, there was a cellular interface separated by a space that corresponds to the subdural (TEM) that we called the dura-arachnoid interface space (SEM), and both surfaces in contact with the (Fig. 2). subdural space were formed by neurothelial cells (Fig. www.neurorgs.com - Unidad de Neurocirugía RGS ANESTH ANALG REGIONAL ANESTHESIA REINA ET AL.