Meninges of the Brain and Spinal Cord
The interior of the cranium and vertebral canal are lined with a layer of the meninges called the dura mater – which is actually separated from the vertebral column by the epidural space, with only a “potential” epidural space between the cranium and this layer of the meninges. The surface of the brain and spinal cord are covered with another layer of the meninges called pia mater. Between these two layers of the meninges are two spaces – the subdural space and subarachnoid space, and a third layer of the meninges called the arachnoid mater.
The subdural space – which is a potential site for haemorrhaging blood, in subdural haematoma (see clinical correlation below), intercept the arachnoid mater and dura mater, while the subarachnoid space lies beneath the arachnoid mater (hence the name “sub”-arachnoid) and between it (arachnoid mater) and the pia mater.
The meninges (singular: meninx) is a Greek word for membrane. Meninges are connective tissue membranes that line the cranium and vertebral canal, and enclose the central nervous system (CNS-brain and spinal cord). It is made up of three layers, which from outwards to inwards or cranium/vertebrae to brain/spinal cord are:
- dura mater,
- arachnoid mater, and
- pia mater.
Hence the pia mater are the closest to the CNS structures, while the arachnoid and dura mater are farthest from the CNS.
The meningeal coverings provide protection to the brain and spinal cord, accompany blood vessels to and from the CNS tissues, and channel the cerebrospinal fluid (CSF) around the surfaces of the brain and spinal cord.
The meninges also support these blood vessels and contains the CSF, within its subarachnoid space. These are the structures involved in meningitis – an inflammation of the meninges, which if severe, may become encephalitis – an inflammation of the brain.
Additionally, the dura mater is a thick tough fibrous and dense inelastic membrane which is conventionally described as consisting of two layers. The dura mater is considered the “hard meninges or pachymeninx”. The arachnoid mater is connected to the pia mater by many fine filamentous processes or webs (hence its name - “arachnoid”, meaning spiderlike). The pia mater is a thin, highly vascular membrane investing the brain and spinal cord. The arachnoid and pia mater are considered the “soft meninges or leptomeninges (leptomenix)”. The region between the arachnoid and pia mater is the subarachnoid space which is filled with CSF.
Development of the Cranial Meninges
Embryogenesis of the meninges varies across vertebrate species. However, the human cranial meningeal development is still a matter of great controversy.
To the extent of present knowledge, the meningeal coverings of the brain develop from both neural crest-derived cells and a mesenchymal layer of the neural tube, during the early embryonic development. These cells of the mesenchymal layer and neural crest form the primordium of the meninges called primary meninx. The primary meninx differentiates, during embryonic development, into two layers of cells: the pachymeninges – a thick layer and future dura mater, and the leptomeninges which develop into the arachnoid and the pia mater.
Development of the Spinal Meninges
Early in the 4th week of embryonic development, the mesenchyme surrounding the neural tube condenses to form a membrane called the primordial meninx or meninges. The external layer of this membrane thickens to form the dura mater. The internal layer, the pia-arachnoid which is composed of the pia mater and arachnoid mater (leptomeninges), is derived from neural crest cells.
Fluid-filled spaces appear within the leptomeninges that soon coalesce to form the subarachnoid space. The development of the pia and arachnoid mater from a single layer is indicated in the adult by arachnoid trabeculae – numerous delicate strands of connective tissue that pass between the pia and arachnoid mater. These arachnoid trabeculae are the webs or fine filamentous processes formed by the arachnoid mater after embryonic development. Cerebrospinal fluid begins to form during the 5th week of development.
The meninges are essentially composed of a series of fibroblasts and/or arachnoid cells, as well as varying amounts of extracellular materials, fibres, and fluid-filled cisterns (openings or spaces).
The dura mater is a rigid but simple covering of the brain as well as an internal covering of the calvaria. It forms venous sinuses that drain blood and CSF. The border layer of the dura mater contains electron-dense cytoplasm and it is also characterized by:
- extracellular cisterns containing fuzzy amorphous materials,
- devoid of connective tissue fibres,
- contains a good number of cell junctions, and
- lacks tight junctions.
Dura mater is composed of greater (periosteal) and lesser (meningeal) laminae, which are formed mainly of collagen fibres that are aligned differently. A small amount of elongated, flattened fibrocytes are intermingled with large number of extracellular collagen fibrils.
There are four areas of infoldings (reflections) in the dura mater, and at these points, numerous arachnoid villi can be found. These areas of infoldings are:
- the falx cerebri,
- the tentorium cerebelli,
- the falx cerebelli, and
- the diaphragma sellae.
The arachnoid contains mechanical strong, flexible, and functional structures, having the following three distinct roles:
- serve as a covering for the pulsatile brain,
- a pathway of the CSF circulation, and
- an anatomical structure for CSF absorption.
For these purposes, the arachnoid cells form numerous morphologically distinct cell junctions, intense cell to cell communications, pinocytotic activity, extracellular cisterns, and specialized segments such as arachnoid villi (arachnoid granulations).
The outer part of the arachnoid mater forms the arachnoid barrier layer, which is comprised of closely packed cells like those of an actual covering membrane. While the inner part of the arachnoid forms the arachnoid trabeculae which is steeped in the CSF of the subarachnoid space, to support the arachnoid barrier layer. Arachnoid barrier cells have a large, polygonal, and electron-lucent cytoplasm with an oval nucleus, showing a marked contrast to the long and flattened dural border cells.
The arachnoid barrier layer is composed of layer of cells with numerous cell junctions, and the presence of many tight junctions which serve as a barrier to the CSF.
This is a layer of cells with long, flattened processes forming the innermost part of the meninges. Pial cells show similarities with arachnoid trabecular cells and have close contact with their attenuated processes. They are composed of a smooth-surfaced, thin layer of cells joined by desmosomes and gap junctions, but no tight junctions are observed.
The gross features of the meninges will be discussed under each of its layers as follows:
This layer is conventionally described as consisting of an outer and an inner layer. The outer layer is often called the endosteal layer (periosteal dura), and the inner, the fibrous or meningeal layer (meningeal dura). However, there is no need for these somewhat confusing alternatives; all are fibrous and all are part of the meninges. This two layer concept is to some extent, false because the outer layer is actually the periosteum which invests the surface of the cranium (just like periosteum invests the surface of any bone). The inner layer of the dura mater is very different; it consists of a dense, strong fibrous membrane, which is really the dura mater proper. The middle meningeal artery and middle meningeal veins lie between the periosteal dura and meningeal dura.
There are folds or reflections of the inner layer (meningeal layer) which project into the cranial cavity. One such fold, the tentorium cerebelli roofs in the posterior cranial fossa; another forms the falx cerebri, lying in the midline between the two cerebral hemispheres (fig.3). The falx cerebelli and diaphragma sellae are smaller derivatives of the inner dura layer; all four folds are described as follows:
This is the largest of all the folds. It is sickle-shaped and lies in the midline between the two cerebral hemispheres. Its front end is narrow and is attached to the internal frontal crest and crista galli. Posteriorly it is broad and blends with the upper surface of the tentorium cerebelli. In the upper fixed margin of the falx cerebri, runs the superior sagittal sinus, while in the lower free margin runs the inferior sagittal sinus. The straight sinus runs along its attachment to the tentorium cerebelli.
This is a vertical infolding that lies inferior to the tentorium cerebelli, separating the cerebellar hemispheres. It is a small, sickle-shaped septum (fold). It attaches (anteriorly) to the occipital crest, and its posterior free margin contains the occipital sinus.
This is the second largest septum or fold of the meningeal dura. It is crescent-shaped and forms a roof over the posterior cranial fossa. It covers the upper surface of the cerebellum and supports the occipital lobes of the cerebral hemispheres. On the anterior aspect of the tentorium cerebelli, there is a gap – the tentorial notch, for the passage of the midbrain.
Three sinuses related to the tentorium cerebelli are:
- Straight sinus – which runs along the attachment of the tentorium to the falx cerebelli,
- Superior petrosal sinus – which runs along the attachment of the tentorium to the petrous bone, and
- Transverse sinus – running along the attachment of the tentorium to the occipital bone.
Diaphragma sellae (Sellar diaphragm)
This is the smallest folding covering the pituitary gland and sella turcica. It is a circular-shaped fold that forms the roof of the sella turcica, and has a small opening in its centre that allows the passage of the stalk of the pituitary gland.
These four septa sent inwards, by the meningeal dura, divide the cranial cavity into freely communicating spaces lodging the subdivisions of the brain. They primarily function to restrict the rotatory displacement of the brain.
On the spinal cord, the spinal dura mater or theca is a prolongation of the inner layer of the dura mater of the posterior cranial fossa. It extends downwards through the foramen magnum to the level of S2 vertebra, thus extending below the level of spinal cord termination L1, and ensuring sufficient protection of the spinal cord by allowing space below the spinal cord level for CSF collection during a procedure like “Lumbar Puncture”. Superiorly the spinal dura mater is attached firmly to the tectorial membrane and to the posterior longitudinal ligament on the body of the axis vertebra (second cervical vertebra – C2), but it lies freely in the spinal canal. It is separated from the spinal canal by a layer of fat in which lies the internal vertebral venous plexus. It is also pierced by anterior and posterior roots of the spinal nerves.
Arachnoid mater and Subarachnoid space
This layer of the meninges consists of an impermeable delicate membrane that is reinforced by the inner layer of the dura mater. However, between this impermeable membrane part of the arachnoid mater and the inner layer of the dura mater (meningeal dura), there is a space called the subdural space – containing lymph (a thin film of tissue fluid). Vessels and nerves pierce the dura and arachnoid mater both at the same place, and never run along between the two membranes.
Lying beneath the arachnoid, is the subarachnoid space. Separating the arachnoid and pia mater in this space are web-like strands called arachnoid trabeculae which also connects both mater. In the spinal part of the subarachnoid space, the arachnoid trabeculae are condensed into a thin posterior midline lamina that forms the incomplete posterior median septum.
The subarachnoid space provides a pathway for the circulation and absorption of the CSF after its escape from the 4th ventricle.
Structures connecting the surface of the brain with foramina necessarily pass through the subarachnoid space. Thus all the cranial nerves and roots of the spinal nerves traverse the space, as well as all the arteries and veins of the brain and spinal cord. The space extends down to the termination of the spinal arachnoid and dura at the level of S2 vertebra.
In certain areas, the arachnoid herniates (protrudes) through little holes in the dura mater into the venous sinuses. Such herniae are the arachnoid villi; through their walls, the CSF “oozes” (is absorbed) back into the blood stream. In adult, these arachnoid villi are aggregated into visible clumps called the arachnoid granulations (Pacchionian bodies).
Resulting from the contours of the brain and cranium, spaces form in the subarachnoid. These spaces form the subarachnoid cisterns. The largest of these cisterns is the cerebellomedullary cistern (cisterna magna). Others are the pontine cistern, interpeduncular cistern and the chiasmatic cistern.
The spinal arachnoid mater shows similar arrangement and features to that of the cranial arachnoid mater. However, the arachnoid trabeculae are condensed into a thin posterior midline lamina that forms an incomplete posterior median septum.
The spinal subarachnoid space is relatively large, accommodating about half of the total volume of the CSF (75ml out of 150ml). It communicates through the foramen magnum with the subarachnoid space of the posterior cranial fossa. In the spinal canal, the space ends at the level of the S2 vertebrae.
This meningeal layer invests the brain and spinal cord as periosteum invests bone. It contains blood vessels, and invests the surface of the CNS to the depths of the deepest fissures and sulci, following the curvatures of the brain surfaces – gyri and sulci. It is prolonged out over the cranial nerves and spinal nerve roots to fuse with their epineurium, and it is invaginated into the substance of the brain by the entering cerebral arteries.
On the spinal cord, the pia mater, as in the cranium, invests the surface of the central nervous system. It clothes the spinal cord and enters to line the anterior median sulcus. It is prolonged over the spinal nerve roots and blends with their epineurium. It is projected below the apex of the conus medullaris, whence it extends as the filum terminale to perforate the spinal theca (spinal dura mater) at the level of S2 vertebra.
The pia mater projects laterally, on each side of the midline, to form the denticulate ligament.
The blood supply to the meninges generally concerns the blood supply to the outer layer of the dura mater (periosteal dura) rather than the inner layer of the inner layer of the dura (meningeal dura), arachnoid mater, or the pia mater. These two meningeal layers and the inner layer of the dura mater, require very little blood to derive nourishment. All the arteries and veins of the meninges lie between the two layers of the dura mater.
In the supratentorial part of the periosteal dura, the arterial supply is by the:
- middle meningeal artery
In the anterior cranial fossa, the arterial supply includes:
- the meningeal branches of the ophthalmic artery
- the meningeal branches of the anterior ethmoidal artery
- the meningeal branches of the posterior ethmoidal artery
Over the cavernous sinus, the periosteal dura is supplied by:
- meningeal branches of the internal carotid artery
- the accessory meningeal artery
In the middle cranial fossa, it is supplied by:
- the ascending pharyngeal artery
- the anterior (frontal) and posterior (parietal) branches of the middle meningeal artery
The middle meningeal artery is a branch of the maxillary artery. It is the most clinically significant arterial supply of the meninges because of its location in the extradural space, and the proximity of its frontal (anterior) branch to the pterion (the point of the cranium where the parietal and temporal bones meet the greater wing of the sphenoid bone) of the skull, making it susceptable to damage in head injuries.
In addition, the frontal branch also lie over the precentral gyrus, and as a result, haemorrhage from it causes pressure on the motor area.
In the posterior cranial fossa, the cranial periosteal dura is supplied by:
- meningeal branches of the vertebral artery
- occipital arteries
- ascending pharyngeal arteries
Like the cranial meninges, the pia mater, arachnoid mater and meningeal dura of the spinal meninges require very little nourishment from blood. Hence the spinal theca is supplied by:
- meningeal branches of the vertebral artery
The venous return of the dura mater is by the middle meningeal veins which accompany the branches of the middle meningeal artery.
The nerve supply of the dura mater is as described below.
In the supratentorial part, it is innervated by:
In the anterior cranial fossa, its nerve supply includes:
- the ophthalmic division of the trigeminal nerve (nasociliary branch)
- the anterior ethmoidal nerve
- the posterior ethmoidal nerve
In the middle cranial fossa, the nerve supply includes:
- meningeal branches of the ophthalmic, maxillary and mandibular divisions of the trigeminal nerve
- tentorial branches of the ophthalmic and maxillary divisions of the trigeminal nerve
In the posterior cranial fossa, it is supplied by: