Chapter 7
The Meninges
Compartments and Herniation Syndromes
Arachnoid Trabeculae and the Subarachnoid Space
The human nervous system is extremely delicate and lacks the internal connective tissue framework seen in most organs. For protection, the brain and spinal cord are each encased in a bony shell, enveloped by a fibrous coat, and delicately suspended within a fluid compartment. In the living state, the nervous system has a gelatinous consistency, but when treated with fixatives, it becomes firm and easy to handle.
OVERVIEW
The brain and spinal cord are surrounded by the skull and vertebral column, respectively. With the exception of the intervertebral foramina, through which the spinal nerves and their associated vessels pass, and the foramina in the skull, which serve as conduits for arteries, veins, and cranial nerve roots, this bony encasement is complete. The membranous coverings of the central nervous system (CNS), the meninges, are located internal to the skull and vertebral column. The meninges (1) protect the underlying brain and spinal cord; (2) serve as a support framework for important arteries, veins, and sinuses; and (3) enclose a fluid-filled cavity, the subarachnoid space, that is vital to the survival and normal function of the brain and spinal cord.
The presence of this bony and meningeal encasement of the CNS is a double-edged sword. Although these structures offer maximum protection, they can be very unforgiving in the case of trauma or in a disease process. For example, growth of a tumor creates a mass that will increase intracranial pressure and compress or displace various portions of the brain. Something has to give inside the skull when a space-occupying lesion develops, and it is the delicate tissue of the brain that gives. The neurologic deficits that result depend on the location of the mass, the rapidity with which it enlarges, and which parts of the brain are damaged.
DEVELOPMENT OF THE MENINGES
The meninges develop from cells of the neural crest and mesenchyme (mesoderm), which migrate to surround the developing CNS between 20 and 35 days of gestation (Fig. 7-1A-C). Collectively these neural crest and mesodermal cells form the primitive meninges (meninx primitiva). At this stage, no obvious spaces (venous sinuses, subarachnoid space) are present in the meninges. Between 34 and 48 days of gestation, the primitive meninges differentiate into an outer, more compact layer called the ectomeninx and an inner, more reticulated layer called the endomeninx (Fig. 7-1D). As development progresses (45 to 60 days of gestation), the ectomeninx becomes more compact and spaces appear in this layer that correlate with the positions of the future venous sinuses. Concurrently, the endomeninx becomes more reticulated and the spaces that appear in its inner part correspond to the subarachnoid spaces and cisterns of the adult. In general, the ectomeninx will become the dura mater and the endomeninx will form the arachnoid mater and pia mater (the leptomeninges) of the adult nervous system (Fig. 7-1D). By the end of the first trimester, the general plan of the meninges is established.
One developmental defect associated with closure of the neural tube and formation of the meninges in the lumbosacral area is the congenital dermal sinus (also called just dermal sinus) (Fig. 7-1E). This defect is caused by a failure of the ectoderm (future skin) to completely pinch off from the neuroectoderm and the primitive meninges that envelop it. As a result, the meninges are continuous with a narrow, epithelium-lined channel that extends to the skin surface (Fig. 7-1E). Dermal sinuses are sometimes discovered in young patients who have recurrent but unexplained bouts of meningitis. These lesions are surgically removed, and recovery is usually complete.
The ectomeninx around the brain is continuous with the skeletogenous layer that forms the skull. This relationship is maintained in the adult, in whom the dura is intimately adherent to the inner surface of the skull. In the spinal column, the ectomeninx is also initially continuous with the developing vertebrae. However, as development proceeds, the spinal ectomeninx dissociates from the vertebral bodies. A layer of cells remains on the vertebrae to form the periosteum, and the larger part of the ectomeninx condenses to form the spinal dura. The intervening space becomes the spinal epidural space (Fig. 7-2). This space is essential for the administration of epidural anesthetics.
OVERVIEW OF THE MENINGES
In general, the meninges consist of fibroblasts and varying amounts of extracellular connective tissue fibrils. The fibroblasts of each meningeal layer are modified to serve a particular function.
The human meninges are composed of the dura mater, arachnoid mater, and pia mater (Figs. 7-2 and 7-3). The outermost portion, the dura mater, also called the pachymeninx, is adherent to the inner surface of the skull but is separated from the vertebrae by the epidural space (Fig. 7-2). Around the brain the inner portions of the dura give rise to infoldings or septa, such as the falx cerebri and tentorium cerebelli (Fig. 7-2), which separate brain regions from each other. Major venous sinuses are found at the points where these septa originate. Spinal and cranial nerves, as they enter or exit the CNS, must pass through a cuff of the dura that is continuous with the connective tissue of the peripheral nerve. Blood vessels traverse the dura in similar fashion. Rostrally the dura sac is attached to the rim of the foramen magnum. Caudally the sac ends at about the level of the second sacral vertebrae and is attached to the coccyx by the filum terminale externum (or dural part of the filum terminale) (Fig. 7-2).
The inner two layers of the meninges, the arachnoid mater and the pia mater (Figs. 7-2 and 7-3), are collectively known as the leptomeninges. This term is also commonly used in clinical medicine (as in leptomeningeal cysts and leptomeningitis). The arachnoid is a thin cellular layer that is attached to the overlying dura but, with the exception of the arachnoid trabeculae, is separated from the pia mater by the subarachnoid space. The arachnoid around the brain is directly continuous with the arachnoid lining the inner surface of the spinal dura (Fig. 7-2). Consequently, the spinal and cerebral subarachnoid spaces are also directly continuous with each other at the foramen magnum. The subarachnoid space contains cerebrospinal fluid (CSF) and vessels and is bridged by fibroblasts of various sizes and shapes that collectively form the arachnoid trabeculae. The arachnoid is avascular and does not contain nerve fibers.
The pia mater is located on the surface of the brain and spinal cord and closely follows all their various grooves and elevations (Figs. 7-2 and 7-3). Around the spinal cord, the pia mater contributes to the formation of the denticulate ligaments and the filum terminale internum (or pial part of the filum terminale) (Fig. 7-2).
DURA MATER
Periosteal and Meningeal Dura
The dura mater (pachymeninx) is composed of elongated fibroblasts and copious amounts of collagen fibrils (Fig. 7-3). This membrane contains blood vessels and nerves and is generally divided into outer (periosteal), inner (meningeal), and border cell portions. There is no distinct border between periosteal and meningeal portions of the dura (Fig. 7-3). Fibroblasts of the periosteal dura are larger and slightly less elongated than other dural cells. This portion of the dura is adherent to the inner surface of the skull, and its attachment is particularly tenacious along suture lines and in the cranial base. In contrast, the fibroblasts of the meningeal dura are more flattened and elongated, their nuclei are smaller, and their cytoplasm may be darker than that of periosteal cells. Although cell junctions are rarely seen between dural fibroblasts, the large amounts of interlacing collagen in periosteal and meningeal portions of the dura give these layers of the meninges great strength.
Dural Border Cell Layer
The innermost part of the dura is composed of flattened fibroblasts that have sinuous processes. Collectively these cells form the dural border cell layer (Fig. 7-3). The extracellular spaces between the flattened cell processes of dural border cells contain an amorphous substance but no collagen or elastic fibers. Cell junctions (desmosomes, gap junctions) are occasionally seen between dural border cells and cells of the underlying arachnoid.
Because of its loose arrangement, enlarged extracellular spaces, and lack of extracellular connective tissue fibrils, the dural border cell layer constitutes a plane of structural weakness at the dura-arachnoid junction. This layer is externally continuous with the meningeal dura and internally continuous with the arachnoid. Consequently, bleeding into this area of the meninges will disrupt and dissect open the dural border cell layer rather than invade the overlying dura or the underlying arachnoid. In the normal (and healthy) human, there is not a naturally occurring, or preexisting, space at the dura-arachnoid interface (Fig. 7-3).
Blood Supply
The arterial supply to the dura of the anterior cranial fossa originates from the cavernous portion of the internal carotid, the ethmoidal arteries (via the ethmoidal foramina), and branches of the ascending pharyngeal artery (via the foramen lacerum). The middle meningeal artery serves the dura of the middle cranial fossa and may be compromised when there is trauma to the skull. It is a branch of the maxillary artery and enters the skull through the foramen spinosum. The accessory meningeal artery (via the foramen ovale) and small branches from the lacrimal artery (via the superior orbital fissure) also serve the dura of the middle fossa. The dura of the posterior fossa is served by small meningeal branches of ascending pharyngeal and occipital arteries and by minute branches of the vertebral arteries.
The spinal dura is served by branches of major arteries (such as vertebral, intercostal, and lumbosacral) that are located close to the vertebral column. These small meningeal arteries enter the vertebral canal via the intervertebral foramina to serve the dura and adjacent structures.
Nerve Supply
The nerve supply to the dura of the anterior and middle fossae is from branches of the trigeminal nerve, Ethmoidal nerves and branches of the maxillary and mandibular nerves innervate the dura of the anterior fossa; the dura of the middle fossa is served mainly by branches from the maxillary and mandibular nerves. The dura of the posterior fossa receives sensory branches from dorsal roots C2 and C3 (and from C1 when this root is present) and may have some innervation from the vagus nerve. The tentorial nerve, a branch of the ophthalmic nerve, courses caudally to serve the tentorium cerebelli. Autonomic fibers to the vessels of the dura originate from the superior cervical ganglia and simply follow the progressive branching patterns of the vessels on which they lie.
Nerves to the spinal dura originate as recurrent branches of the spinal nerve located at that level. These delicate strands pass through the intervertebral foramina and are distributed to the spinal dura and to some adjacent structures.
Dural Infoldings and Sinuses
The periosteal dura lines the inner surface of the skull and functions as its periosteum. The meningeal dura is continuous with the periosteal dura but draws away from it at specific locations to form the dural infoldings (or reflections). The largest of these is the falx cerebri (Figs. 7-4 and 7-5A). It is attached to the crista galli rostrally, to the midline of the inner surface of the skull, and to the surface of the tentorium cerebelli caudally. The falx cerebri separates the right hemisphere from the left. The superior sagittal sinus is found where the falx cerebri attaches to the skull, the straight sinus where it fuses with the tentorium cerebelli, and the inferior sagittal sinus at its free edge (Fig. 7-4). Many large superficial veins located on the surface of the cerebral hemispheres empty into the superior sagittal sinus.
The tentorium cerebelli is the second largest of the dural infoldings (Figs. 7-4 and 7-5B, C). It attaches rostrally to the clinoid processes, rostrolaterally to the petrous portion of the temporal bone (location of the superior petrosal sinus), and caudolaterally to the inner surface of the occipital bone and a small part of the parietal bone (location of the transverse sinus) (Figs. 7-4 and 7-5B, C). The tent shape of the tentorium divides the cranial cavity into supratentorial (above the tentorium) and infratentorial (below the tentorium) compartments (Fig. 7-5B; see also Fig. 7-10). The supratentorial compartment is divided into right and left halves by the falx cerebri (Fig. 7-5A, B). The sweeping edges of the right and left tentoria, as they arch from the clinoid processes to join at the straight sinus, form the tentorial notch (Fig. 7-6). The occipital lobe is above the tentorium, the cerebellum is below it, and the midbrain passes through the tentorial notch.
Located below the tentorium cerebelli on the midline of the occipital bone is the falx cerebelli (Fig. 7-4). This small dural infolding extends into the space found between the cerebellar hemispheres and usually contains a small occipital sinus.
The smallest of the dural infoldings, the diaphragma sellae (Figs. 7-4 and 7-6), forms the roof of the hypophyseal fossa and encircles the stalk of the pituitary. The cavernous sinuses are found on either side of the sella turcica, and the anterior and posterior intercavernous sinuses are found in their respective edges of the diaphragma sellae.
It is emphasized that venous sinuses are endothelium-lined spaces
