Is The Leptomeningeal Space As The Anterior Subarachnoid Space

lateral crus cerebri

Abbreviations: A1 /A2= lst/2nd segment of Anterior Cerebral Artery; ACA= Anterior Cerebral Artery; AChorA= Anterior Choroidal Artery; ACommA= Anterior Communicating Artery; ACommV= Anterior Communicating Vein; AICA= Anterior Inferior Cerebellar Artery; ICA= Internal Carotid Artery; MCA= Middle Cerebral Artery; MCV= Middle Cerebral Vein; MedPostChorA= Medial Posterior Choroidal Vein; Pl-P3= 1st through 3rd segments of Posterior Cerebral Artery; PCA= Posterior Cerebral Artery; PCommA= Posterior Communicating Artery; PICA= Posterior Inferior Cerebellar Artery; PSA= Posterior Spinal Artery; SCA= Superior Cerebral Artery.

The arrangement of the layers of the spinal leptomeninges differs significantly from that of the cerebral leptomeninges because of the presence of an actual epidural space in the spine. The epidural space is found caudal to the attachment of the dura to the foramen magnum13 and contains the epidural veins, lymphatics, and adipose tissue.5

Attachment of the pia to the arachnoid in the spine is not accomplished by the random arrangement of arachnoid trabeculae, as in the cranium. Rather, there is a regular arrangement of septae. The longitudinal midline dorsal septum is one of these septae. It is a condensation of arachnoid, which extends from the dorsal midline arachnoid, encloses the mid-dorsal vein, and attaches to the subadjacent pia. In cases where the middorsal vein is tortuous, the midline dorsal septum is tortuous as well, following the vein in its contours. This midline dorsal septum extends from mid-cervical levels to upper lumbar levels; rostral and caudal to these levels the septum becomes progressively more fenestrated until it is no longer recognizable.14

The dorsolateral septae are paired attachments of arachnoid which extend from the dorsal root entry zone, envelop the dorsal rootlets and then follow the rootlets laterally. This attachment continues into the root sleeve, where it may distinguish the dorsal rootlets from the ventral rootlets, the latter having no arachnoid covering. The dorsolateral septae are most obvious at thoracic and low cervical levels.14

Midway between the dorsal root entry zone and the ventral roots exists a lateral condensation of pia mater referred to as the dentate or denticulate ligament (dentate meaning "sawlike" in Greek). The pial cells of the dentate ligament surround thick collagen bundles. These bundles blend with the subpial collagen surrounding the spinal cord medially while laterally, the dentate attaches to the collagenous dura.15 The dentate ligaments occur at regular intervals and generally extend rostrally from the entry of the vertebral artery into the subarachnoid space to the caudal T12/L1 area.14

The pial covering of the anterior spinal artery forms an irregular longitudinal band referred to as the linea spendens. This condensation of pia mater does not attach to the arachnoid.13,14 The conus medullaris gives rise to a thin ligamentous extension of pia covered by arachnoid cells. This extension is referred to as the filum terminale internum (or simply filum terminale). A segment of the filum terminale attaches and passes through the caudal-most segment of dura, which in turn is attached to the coccyx; after passing through the dura it is referred to as the filum terminale externum.H'1,5

2.2 The fine structure of the arachnoid membrane

Ultramicroscopic examination of the arachnoid has revealed two components making up this layer: an outer layer, often referred to as the arachnoid barrier cell layer; and an inner layer, often referred to as the arachnoid trabeculae (Fig. 1).

Trabeculae Found Brain
Figure 1. The fine structure of the arachnoid membrane.

The arachnoid barrier cell layer is a layer of two to three tiers of flattened cells. These cells have a large, oval- to spindle-shaped nucleus, multiple cytoplasmic processes, scant mitochondria, small rough endoplasmic reticulum and a poorly developed Gogli apparatus.5'16'17 These cells are located under the dural border cell layer of the dura mater. A basement membrane underlies the arachnoid barrier cell layer and separates this layer from the underlying subarachnoid space.5

The presence of junctional complexes is an important characteristic of the arachnoid barrier cell layer. Numerous zonulae occludens (tight junctions), zonulae adherens, and macula adherens (desmosomes) are found interconnecting cells of this layer. These connections function as the meningeal barrier, which excludes proteins and other large molecules from diffusing from the blood to the CSF in the subarachnoid space.5'18,17 The function of this barrier may be demonstrated by the intravascular introduction of dyes: the dye will stain the dura but not the underlying meningeal layers, the CSF, or the brain parenchyma.17

Occasional intercellular connections (viz. desmosomes) also exist between the cells of the arachnoid barrier cell layer in the cranium and the overlying dura. In contrast, intercellular connections between the cells of the dural layers are infrequent. The lack of these intercellular junctions may explain why extravasated blood collects then not in a "potential" subdural space as implied by many textbooks, but in reality, in an intradural location (i.e. between fine layers of the dura).5 A final interconnection of note exists between the cells of the arachnoid barrier cell layer and the underlying arachnoid trabecular cells. The trabecular cells penetrate the basement membrane to attach to the arachnoid barrier cell via desmosomes.5 The subarachnoid trabeculae cells are found below the arachnoid barrier cell layer traversing the subarachnoid space as thin, web-like chordae. The arachnoid trabeculae cells are more loosely arranged and more flat in appearance than the arachnoid barrier cells. The cells of the trabecular layer also have smaller nuclei, abundant mitochondria, and well-developed Golgi apparatuses and rough endoplasmic reticulum16. Extracellular collagen fibrils are found outside of the cells in this layer.17

As mentioned previously, tight junctions are often present in the intercellular connection between cells of the arachnoid barrier and trabecular layers.5'18. Gap junctions often connect cells within the arachnoid trabecular layer. The extensive gap junctions allow the arachnoid cells to function together to allow the passage of small molecules from cell to cell.18

2.3 The fine structure of the pia mater

The cells of the pia mater are modified fibroblasts similar to the cells of the arachnoid membrane. Their morphology is often undistinguishable from that of the arachnoid cells.18'19 The pial layer varies in thickness from one to three cells thick.17 In the cauda equina the pia may be fenestrated20 leaving the basement membrane of the underlying glial limitans of the parenchyma exposed to the subarachnoid space.18

Two layers of the spinal pia were distinguished by Key and Retzius (1875); this distinction has only rarely been referred to by subsequent authors. The outer component has been called the epipial6 or intermediate leptomeningeal layer 5 which is a vascular layer present only in the spinal cord. It covers the collagenous core of the denticulate ligament laterally and composes the linea splendens anteriorly.6 The intimal layer of pia is an avascular layer found in both the spinal cord (as the inner component) and the brain. In contrast to the overlying epipial layer, it is adherent to the brain and spinal cord throughout all its contours. Blood vessels pierce the intimal pia as they pass into the brain or spinal cord.15,6 It has been proposed that the vascular epipial layer represents the contribution of mesenchyme to the pia while the avascular intimal layer represents the contribution of the neural crest.6

A subpial space of variable thickness exists between the pia and the basement membrane of the glial limitans (outer glial layer of the brain and spinal cord). This space contains collagen fibrils.19 Pial cells are often joined to arachnoid trabecular cells with desmosomes.5

2.4 Blood vessels in the subarachnoid space

Blood vessels in the subarachnoid space travel along the outer surface of this space, often suspended from the overlying trabecular layer by chordae composed of arachnoid trabecula cells.20 It had been previously thought that the pia mater follows the arteries and arterioles for some short distance as they descend into the brain parenchyma. The perivascular space between the descending vessel and the pia, often referred to as the Virchow-Robin space, was thought to communicate with the subarachnoid space. Scanning electron microscopy, however, has revealed that the pia actually surrounds the vessel as it travels through the subarachnoid space but does not accompany the vessel as it descends into the brain parenchyma. Instead, the pia surrounding the vessel spreads out over the pia which is covering the surface of the brain, effectively occluding the perivascular space from the subarachnoid space20 (Fig. 2). Thus the Virchow-Robin space communicates with the brain extracellular space rather than the subarachnoid space.

A layer of smooth muscle and extracellular matrix separates the pia from the endothelial cells.18 Similar to the arachnoid cells of the barrier layer, the endothelial cells are interconnected by tight junctions.17

Extracellular Matrix Brain ImageEpendymal Cells Development
Figure 2. Blood vessels in the subarachnoid space.

2.5 Ependyma

Ependymal cells are found as a monolayer which lines the third and fourth ventricles and the central canal of the spinal cord. Their cell morphology varies, ranging from squamous to cuboid to columnar. Another characteristic is their many cilia. These cilia are associated with a basal body and microtubules with the "9+2" arrangement typical of cilia elsewhere. The nucleus of the cell is oval and regular with an eccentric nucleolus. Organelles such as Golgi and mitochondria are often found in the apical portion of the cell. Ependymal cells are interconnected with fascia adherentes (extensive forms of zonulae adherentes) and gap junctions [19].

The primary function of the ependyma may be movement of the CSF caused by beating of the cilia. These cells may also be responsible for trapping foreign cells or microorganisms, and in regenerating ependymal cells. Ependymal cells in the third ventricle may be involved in signaling or transporting molecules to the adenohypophysis.18

2.6 Tanycytes and macrophages

Tanycytes are found in clusters in the walls of the third ventricle and cerebral aqueduct, in the floor of the fourth ventricle, and in the cervical spinal canal. Clusters of tanycytes are often associated with circumventricular organs, namely the median eminence, the area postrema, the subcommissural organ, and the pineal gland.18

In contrast to the ependymal cells, tanycytes have many microvilli and few cilia. Their nuclei are denser and more elongated than those of the ependymal cells. These cells have three portions: 1) a somatic portion, 2) a neck portion, and 3) a tail portion. The somatic portion is the segment of the cell which rests in the ependymal layer; this section has many lateral cytoplasmic processes. The neck is the portion of the cell which extends into the periventricular neuropil to contact blood vessels. The tail portion features processes with end-feet which course through the hypothalamus to contact fenestrated blood vessels or pial surfaces.21 The connection that the tanycyte makes between the ventricle and the capillary has led some to conjecture that the tanycyte functions in the transport of hypophysiotropic hormones. However the research supporting this may be inconclusive.

Fixed macrophages are also present in the arachnoid border layer - these cells are sometimes referred to as Kolmer or epiplexus cells when associated with the choroid plexus. They contain many membrane-bound inclusions and variable vacuoles; they lack cytoplasmic processes.18

2.7 Choroid plexus

The term choroid plexus is most commonly used to refer to the ependymal-derived epithelium which lines the roof of the third and fourth ventricles and the lateral walls of the lateral ventricles. Originally, however, the term choroid plexus referred only to the vasculature underlying this epithelium, while the term tela choroidea was used to refer to the choroid plexus vasculature and the overlying epithelium together.18 Development of the choroid plexus begins as pia. Blood vessels invade the wall of the ventricles, creating folds covered by pseudostratified columnar epithelium. These folds lobulate and eventually the cells become cuboidal-to-squamous in morphology.

The cells feature pale, round central nuclei and apical mitochondria. The luminal surface is lined with both irregular, tightly-packed microvilli and irregular cilia with a "9+2" arrangement of microfilaments. Choroid epithelial cells are joined together with "leaky" tight junctions similar to those found in the gallbladder. Underneath the superficial monolayer of choroidal cells, occasional immature cells can be found. These cells have been shown to take up tritiated thymidine. In primates, renewal of the entire monolayer of choroid has been estimated to occur every one to three years.19

The underlying vasculature of the choroid plexus is notable for its fenestrated, thin-walled, relatively large-diameter capillaries. The arterial supply to the choroid plexus of the lateral ventricles is supplied via the anterior and posterior choroidal arteries; the anterior is a segment directly derived from the internal carotid while the posterior is a branch of the posterior cerebral artery. The choroid plexus of the third ventricle is supplied by choroidal branches of the posterior cerebral artery, while the choroid plexus of the fourth ventricle is supplied by the posterior inferior cerebellar artery with possible supplementation from the anterior inferior cerebellar artery and the internal auditory artery. The thalamostriate and internal cerebral veins drain the majority of the blood from the choroid plexus of the lateral and third ventricles; most of the blood from the choroid plexus of the fourth ventricle is drained by the basal vein of Rosenthal.22 The choroid plexus has a rich autonomic innervation supplied by the cervical sympathetic chain and the vagus.21

2.8 Arachnoid villi and granulations

Arachnoid granulations were first illustrated by Vesalius who observed their imprint on the inner surface of the skull. Pacchioni described the structures, but mistakenly thought that they were lymph nodes which irrigated the meninges. Faivre is accredited with correctly proposing that the granulations serve to drain CSF.2

These leptomeningeal structures are often thought of as one-way valves from the CSF compartment to the venous compartment. They are commonly called arachnoid villi when microscopic or arachnoid granulations when macroscopic. The name Pacchionian granulation has been used to refer to large, elaborate arachnoid granulations in horses and in man.23

Harvey Cushing, in his 1901 Mutter lecture, proposed that the arachnoid villi functioned as one-way valves similar to the valves in the lymphatic system {i.e. an "open" system). At the same time, L.H. Weed, a researcher in the Hunterian labs, found no structures which resembled valves upon light microscopic examination of the villi. He found only an intact membrane covering the villi, and proposed that transcellular transportation occurred via pinocytosis {i.e. a "closed" system).24

The advent of electron microscopy led to re-examination of the functional anatomy of the arachnoid villi and helped establish the fact that micropinocytosis does indeed contribute to the unidirectional flow of csr25,26 The presence of unidirectional valves has been found in monkeys and there is some evidence that widened intercellular gaps contribute to the unidirectional flow of CSF in humans as well.27 However, the question of whether the system is "open" or "closed" (or a combination) remains to be answered definitively.

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Responses

  • Duenna
    Is the leptomeningeal space as the anterior subarachnoid space?
    9 months ago

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