Embryo at 27 days post-conception , stage 12, transverse section, hematoxylin and eosin staining. The neural tube is surrounded by a mesenchyme without meningeal differentiation. Courtesy of Pr P. Dechelotte
Descriptive and Topographical Anatomy of the Spinal Meninges in Adults
The spinal meninges consist of three concentric layers that envelop the spinal cord, the filum terminale, and nerve roots.
The Dura Mater
The dura mater is the thick superficial meningeal layer or pachymeninx. It forms a white, inelastic but deformable sheath that grossly follows the contour of the spinal cord, the filum terminale, and nerve roots. Its thickness varies with individuals and decreases over a lifetime ranging from a thin transparent membrane to a thick pearly wall. The spinal dura corresponds to the inner, meningeal layer of the cranial dura with which it is continuous at the foramen magnum . Ventrally, the meningeal layer separates from the periosteal layer at the 3rd cervical vertebral body.
Dural ultrastructure is made of three distinct layers : an outermost fibroelastic layer, a middle fibrous layer, and an innermost cellular layer. The abundance of elastic fibers and the helicoidal arrangement of collagen bundles provide the flexibility and the resistance of the dural sheath that protect the spinal cord during movements .
The Fixation-Points of the Spinal Dura Mater
The spinal dura is poorly vascularized. Arterial supply is performed by thin branches of radicular arteries whose diameter does not exceed 0.5 mm. Dorsally a dense anastomotic network supplied by two cranio-caudal arterial axes contrasts with a loose ventral anastomotic network supplied by a ventro-median arterial axis. This disposition resembles the spinal cord arterial supply. This arterial network displays a metameric disposition with numerous anastomoses. At the dorsal aspect of the thoracic segment, spiral arteries and tufts of capillaries bulge into the epidural space. The significance of these formations is not understood .
The physiology of the connections between dural and spinal cord venous drainages remains unclear, anatomic observations and interventional neuroradiology providing conflicting data. There are generally two veins for one dural artery. At the cervical level they form a meshwork of large venous sinuses that communicate cranially with the basilar plexus and take part in the encephalic venous drainage. Anatomic studies describe radicular veins that collect from both the veins of the dura and the veins of the spinal cord, leave the vertebral canal by intervertebral foramina and drain into the internal vertebral plexus of the epidural space. Valves in radicular veins located just before they pass through the dura may prevent the blood from flowing back to the spinal cord. These can be regarded as a protective mechanism of the spinal cord in situations of hypertension in the vertebral venous system . Reversely, angiographic procedures do not show communications between perimedullary and epidural veins. Perimedullary veins are described as draining upwards through the foramen magnum into the inferior cerebellar veins and the dural sinuses of the posterior cranial fossa. These reports are provided by procedures requiring the patient to be in a supine position. One can assume that the distribution of the venous return may be different in upright positions or during exercise.
The lymphatic drainage of the spinal dura remains poorly understood. Lymphatic vessels were first described following experiments of ink injections into the ventricular or subarachnoid spaces of mammals. They were described as arising near the lateral points of attachment of the ligamenta denticulata, near lumbar vertebral bodies or around subarachnoid recesses . They drain into paravertebral lymph nodes, the thoracic lymphatics into the nodes of the posterior mediastinum, and the lumbo-sacral lymphatics into the nodes of the posterior abdominal wall between psoas major muscles .
In animal models the lymphatic system has been demonstrated to take part in the absorption of cerebrospinal fluid . In humans the participation of the spinal lymphatic pathway remains unknown under physiological conditions but might be significant in the upright position and during exercise. This function could be especially active in neonates whose arachnoid villi become fully functional only after the age of 18 months, and in the elderly where absorptive capacity of cranial arachnoid granulations gradually decreases.
The ventral aspect of the spinal dura is innervated by a dense plexus supplied by sinu-vertebral nerves, the nerve plexus of the posterior longitudinal ligament and the perivascular plexus of radicular arteries. The sinu-vertebral nerve is a ventral branch of the spinal nerve (Fig. 11). It runs cranially and medially between the dorsal longitudinal ligament and the annulus fibrosus ventrally and the ventral aspect of the dura dorsally. These three structures are innervated by the sinu-vertebral nerve [21–24].
The dorsal aspect of the spinal dura is poorly innervated by nerves coming from the ventral dural nerve plexus in the intervals between nerve roots. They do not reach the median part of the dorsal dura. The lack of innervation of the median dorsal dura explains why lumbar punctures are not painful while traversing the dura mater .
Dural nerves are non-myelinated fibers involved in vasomotricity and nociception [22, 25]. The efficacy of epidural blocks performed in anesthesiology could be at least partially related to an action on dural nerves. The innervation of the dorsal longitudinal ligament and the dorsal part of the annulus fibrosus by sinu-vertebral nerves through free endings has suggested their involvement in low back pain syndromes [26, 27] and might explain the efficiency of periradicular infiltrations with anti-inflammatory drugs in this indication.
The Relationships of the Dura Mater with the Leptomeninges and the Spinal Nerves
The inner aspect of the spinal dura is covered by the outer arachnoid layer. Laterally, as the nerve roots pass through the dura mater, the arachnoid mater constitutes subarachnoid recesses around spinal nerves (Figs. 7 and 11). Ventral and dorsal nerve roots traverse the dura by two distinct openings and run toward the intervertebral foramen within two distinct dural sheaths. The roots of the first cervical nerve and the vertebral artery traverse the dura through the same opening (Fig. 8). In the intervertebral foramen nerve roots merge into a spinal nerve covered by a unique dural sheath. The sinu-vertebral nerve runs at the ventral aspect of the spinal nerve, outside the dural sheath, among the venous plexus of the intervertebral foramen (Fig. 11). The spinal nerve and the radicular artery running at its ventral aspect are disposed centrally in the cellulo-adipose tissue of the foramen surrounded by the epidural intervertebral venous plexus. The intervertebral foramen is closed laterally by the fibrous operculum of Forestier (Fig. 11). The area between the fibrous operculum and the nerve dural sheath is the epidural space of the foramen.
The Arachnoid Mater
According to this description, the subarachnoid space, filled with the extra-axial cerebrospinal fluid, takes the room between the superficial barrier cell layer and the pia mater, traversed by the arachnoid trabeculae of the reticular cell layer. On traversing the arachnoid mater blood vessels and nerve roots are sheathed by extensions of the reticular cell layer. Cranially, the subarachnoid space is continuous with the cranial subarachnoid space at the foramen magnum. Around the spinal cord it constitutes the perimedullary space, divided into ventral and dorsal chambers by the ligamenta denticulata (Figs. 8 and 20). In the dorsal chamber, arachnoid trabeculae form a dense network that firmly applies blood vessels against the spinal cord and constitutes medially a sagittal septum, the septum posticum of Schwalbe. An intermediate cell layer interposed between the superficial arachnoid layer and the pia mater to which it is tightly connected, might contribute to this septum . Most developed at the lower cervical and thoracic levels, the septum posticum of Schwalbe connects dorsally to a thickening of the superficial arachnoid layer: the median raphe of Magendie (Fig. 16).
Caudally the subarachnoid space widens into the large lumbosacral terminal cistern that encloses the cauda equina and terminates between the 1st and the 2nd sacral vertebrae. Laterally it follows the nerve roots from the spinal cord to the intervertebral foramina. The two arachnoid layers merge to limit subarachnoid recesses around spinal nerves at the lateral limit of spinal ganglia just before they traverse the fibrous opercula. The abundance of cell debris and activated macrophages in arachnoid recesses  where fine lymph vessels have been described to drain into paravertebral lymph nodes  suggest the subarachnoid recess as an interface between the central nervous system, the cerebrospinal fluid, and CSF immune defense. There is no specific arachnoid vascularization; the vessels are those of the spinal cord.
Subarachnoid spaces can be precisely explored using CT-scan after intrathecal injection of iodinated contrast medium. MRI visualizes the subarachnoid space by the hypersignal of the cerebrospinal fluid in T2-weighted sequences.
Subarachnoid recesses have relationships with bone structures that constitute the safety landmarks of periradicular infiltrations, procedures commonly performed in anesthesiology and rheumatology (Fig. 14). Inadequate site of puncture can lead to CSF leak, nerve-root damage, or intra-arterial administration. Lateral limits of subarachnoid recesses vary according to vertebral segments and subjects. At cervical levels subarachnoid recesses usually stop at the anterior border of the pedicles. At thoracic and lumbar levels they usually do not extend laterally beyond the caudal border of the upper pedicle. To be safe, the tip of the trocar should remain extraforaminal, that is, outside the lateral recess to avoid intrathecal injection, puncture of a radiculomedullary artery, or a vertebral artery in the cervical segment.
Around spinal nerve roots the arachnoid mater has been described to differentiate into arachnoid villi. Morphological similarities with cranial granulations and their relationships with epidural veins have suggested their involvement in CSF reabsorption [34, 35]. Usually located at or medially to subarachnoid recesses, they could be more frequent in thoracic or lumbar regions. In animal models, spinal arachnoid villi have been assessed to be responsible for about 25% of CSF drainage [36, 37]. In humans, their contribution in CSF absorption has never been assessed but might be effective in the first year of life and later on in upright posture or during exercise .
The Pia Mater
The spinal pia mater is a thin areolar tissue closely adhering to the glia limitans (Figs. 6 and 16). Cranially the spinal pia mater continues as the cranial pia mater at the foramen magnum. Caudally it encases the filum terminale below the conus terminalis. Classically considered a pial formation, the filum terminale was demonstrated to originate from apoptotic degeneration of the caudal spinal cord . In human fetuses, the filum terminale is constituted of a connective tissue rich in type III collagen with nerve fascicles, blood vessels, ganglion cells, ependymal, glial, and adipose tissues . The same components are found in adults, but connectives fibers are mostly type I collagen, elastin and elaunin fibers longitudinally arranged in a network of transversal type III collagen fibers . The filum terminale appears a bluish, white formation of about 20 cm long and less than 2 mm wide. Its upper part, located inside the dural sac (filum terminale internum), runs down along the lumbosacral terminal cistern among the roots of the cauda equina. Its lower part, located beyond the dural sac (filum terminale externum), is encased by the filum of the dura mater from the lower border of the first sacral vertebra to the dorsal aspect of the first coccygeal piece. Laterally the pia mater follows nerve roots and spinal nerves, fusing with the arachnoid before commencement of the perineurium.
In the ventral fissure of the spinal cord, the pia mater forms the linea splendens, a dense network of fibrous strands that bridge the two walls of the fissure and wrap the ventral spinal artery (Fig. 6).