Meningeal anatomy

Anatomy of meninges

General

The dura and arachnoid share common precursor cells and remain attached throughout development.

Dural border cell layer is a structurally weak layer at the dura–arachnoid junction.

Sandwiched between the dural layer dense in collagen and the arachnoid layer reinforced with cellular junctions, the dural border cell layer represents the plane of least resistance.
This is where subdural hydromas or sudural haematomas form.

Dura mater

Latin for “hard mother,”

Aka pachymeningeal, or thick layer, (dense connective tissue).
The tenacity of the dura mater stems from its composition of elongated fibroblasts and extensive amounts of extracellular collagen.

The varying orientations of fibers create a matrix of intertwining collagen that provides significant strength.

Layers

General: The dura mater is composed of two distinct layers that remain fused throughout the majority of the cranium.

Periosteal dura

Outer layer

Is highly vascularized

Intimately adherent to the overlying cranium

Functions as its periosteum.

Continuous with the periosteum covering the external cranium at suture lines and neural foramina.

It merges with the periorbital membrane through the optic canal and the superior orbital fissure.

As the cranial nerves exit through their respective cranial foramina, the inner meningeal layer surrounds the nerves as tubular sheaths that will fuse with the epineurium.

Cellular and molecular characteristics

Thicker, with more abundant collagen
Cell characteristic

Large, elongated, somewhat flattened fibroblasts in varying orientations with extensive amounts of intertwining extracellular collagen

Cell organelles

Large amounts of granular endoplasmic reticulum, chromatin-containing nuclei, a Golgi apparatus, ribosomes, mitochondria, fat droplets, filaments

Cell junction

Infrequent

Meningeal dura

Cellular and molecular characteristics

Cell characteristic

Increased amounts of elongated, flattened fibroblasts with long processes and proportionately less collagen

Cell organelles

Condensed cytoplasm with elongated nuclei

Cell junction

Infrequent

Reflects away from the periosteal layer at multiple locations to form the venous sinuses.

Forms dural septa

Falx cerebri
Tentorim cerebelli
Diaphragma sellae
Falx cerebelli

Dura border cells

Located at the dura–arachnoid junction

A distinct cell population

Molecular and cell characteristics

Cell characteristic

Modified, elongated, flattened fibroblasts, with amorphous, nonfilamentous material and no collagen in enlarged extracellular compartment

Instead, the extracellular spaces are filled with irregular cellular processes and an amorphous, nonfilamentous granular material.

Cell organelles

Dense, dark cytoplasm with elongated nuclei

Cell junction

Few cell junctions;
Occasional desmosomes and gap or intermediate junctions with arachnoid layer

The dural border cell layer, therefore, is continuous with the arachnoid layer.

Spinal dura

The external endosteal layer of cranial dura mater ends at the foramen magnum continues as the periosteum of the spinal canal. Thus, the spinal dura mater is composed of the inner or meningeal layer of cranial dura mater.
Caudally, the spinal dura ends at the level of S2 where it becomes a thin cord-like extension (coccygeal ligament or filum terminale) that anchors the dural sac to the sacral periosteum.
The dura mater is attached to the circumference of the foramen magnum and the second and third vertebrae.
It is also attached anteriorly to the posterior longitudinal ligament by the fibrous Bands of Hofmann.
The posterior surface is relatively more mobile and the connective tissue (the meningovertebral ligaments) is less fibrous

Thus, the anterior attachment supports and secures the dura anteriorly in the spinal canal while the posterior surface is allowed greater mobility.

Arachnoid mater

The arachnoid and the pia mater form the leptomeningeal, or thin layer of meninges

Layers

Arachnoid barrier cells

Cell characteristic

Closely packed plump cells without any significant amount of extracellular space
Absence of collagen

Cell organelles

Translucent cytoplasm with prominent Golgi apparatus, numerous mitochondria, large oval-shaped nuclei

Cell junction

Numerous cell junctions, especially desmosomes and tight and gap junctions

Tight junctions among cells strengthen the arachnoid barrier cell layer and render it impermeable to fluids, large molecular weight substance, and even some ions.

Arachnoid trabeculae

Cell characteristic

Loosely organized fibroblasts with long, flattened, irregular processes
Arachnoid trabecular cells

Are specialized fibroblasts with long processes and attachment to the arachnoid barrier layer.
They bridge the subarachnoid space with their long, flattened, irregular processes and may form cellular attachments with pial cells.
Collagen may be found within the trabecular matrix created by the processes in the subarachnoid space.

Cell organelles

Translucent cytoplasm with prominent Golgi

Cell junction

Cell junctions join trabeculae processes to each other and to overlying arachnoid barrier layer

Pia Mater

A thin sheet of flattened fibroblast cells that separates the subarachnoid space from the subpial and cortical perivascular spaces.

The basement membrane of the outer glial layer of the brain and spinal cord, termed glia limitans, separates the pia from the underlying neural tissue to create the subpial space.

These modified fibroblasts of pia form junctional complexes at their margins, rendering the pia impermeable to particulate matter, such as blood.

The pia is reflected from the surface of the brain to surround vessels traveling in the subarachnoid space but does not accompany the vessels into parenchyma.

This arrangement seals the subarachnoid space from the subpial and perivascular spaces, such that subarachnoid blood does not enter the subpial space.

Molecular and cell characteristics

Cell characteristic

Flattened fibroblasts

Cell organelles

Few organelles with translucent cytoplasm

Cell junction

Few cell junctions, underlying basement membrane between pia and brain

Feature	Virchow–Robin Space (VRS)	Pial/Subpial Space
Lining	Pial cells; forms sleeves with vessels	Underlying pia mater, non-sleeve
Location	Surrounds penetrating arteries/arterioles	Beneath pia, brain surface
Continuity	May connect to subpial space	Communicates with VRS and subarachnoid
Imaging	Visible as CSF-like spaces on MRI	Not typically seen as discrete spaces
Function	Fluid drainage, glymphatic pathway	Surface protection, barrier

A, The subarachnoid space is delimited by the arachnoid mater and the pia mater. The pia mater is composed of a single layer of fibroblasts supported by a thin fibrous tissue matrix and underlain by the subpial space. This space is separated from the cerebral parenchyma by the glia limitans.
B, Arteries passing from the subarachnoid space into the cerebral cortex are surrounded by a perivascular sheath composed of the pial membrane, which is adherent to the glia limitans and forms the inner lining of the PVS. This pial lining becomes incomplete and then disappears at the level of capillaries. Capillaries have no PVS, because their ECBM is fused with the glia limitans (itself formed by astrocyte foot processes embedded in the parenchymal basal lamina). Postcapillary venules (PCVs) are surrounded by a PVS that is delimited by ECBM and the glia limitans; there is no intact pial membrane, but small clusters of pial cells adherent to the vessels are present. The PVS is in continuity with the subpial space (A).
ECBM indicates endothelial basement membrane; PBM, parenchymal basement membrane; PF, pial fenestration; and PVS, perivascular space.

Arachnoid granulations

Aka: Arachnoid villi/Pacchionian bodies

Arachnoid villi are microscopic,
Arachnoid granulations are visible to the naked eye
Pacchionian bodies are especially large, elaborate complexes.

Arachnoid villi are specialised segments of the meninges that project into the sinuses and major venous structures

Function:

Absorption of CSF through both passive and active mechanisms.

A fibrous capsule reflected from the surrounding dura covers arachnoid villi except at the apices, where the underlying arachnoid cell layer and specialised arachnoid cap cells are exposed to the venous blood of the sinus.

These cells are highly metabolically active and are involved in the resorption of CSF.

Arachnoid cap cells are derived from the outer portions of the endomeninx

Considered the cells of origin of meningiomas.

Located throughout the central nervous system

Found in greatest concentration

Adjacent to the major sinuses, large cerebral veins, and basilar plexus
Around the crista galli
Over the cribriform plate
At the exit foramina of cranial nerves II through VII and IX through XII

Ventricles
Sylvian fissure
Pineal region

Illustration of the fine structure of an arachnoid granulation and its relation to the superior sagittal sinus

Meningeal vascular supply

Source	Parent Vessel	Branch	Area of Supply
ECA	Ascending pharyngeal artery	Carotid branch	Dura of foramen lacerum and carotid canal
ㅤ	ㅤ	Jugular branch	Dura of jugular foramen, walls of inferior petrosal sinus, jugular bulb and inferior sigmoid sinus, dura of posterior petrosal surface
ㅤ	ㅤ	Hypoglossal branch	Dura of foramen magnum, inferolateral cerebellar fossa
ㅤ	Occipital artery	Jugular branch	Dura of jugular foramen, walls of inferior petrosal sinus, jugular bulb and inferior sigmoid sinus, dura of posterior petrosal surface
ㅤ	ㅤ	Hypoglossal branch	Dura of foramen magnum, inferolateral cerebellar fossa
ㅤ	ㅤ	Mastoid branch	Dura over posterior petrosal surface, lateral and paramedial cerebellar fossa
ㅤ	ㅤ	Parietal emissary branch	Dura over posterior parietal convexity
ㅤ	Middle meningeal artery	Petrosal branch	Lateral wall of cavernous sinus, dura over posteromedial floor of middle fossa, medial tentorium insertion along petrous ridge
ㅤ	ㅤ	Anterior division	Dura over frontal and anterior parietal convexity, walls of superior sagittal sinus and falx, dura over lateral portion of anterior and middle fossa
ㅤ	ㅤ	Posterior division	Dura over posterolateral floor of middle fossa; lateral tentorium insertion along petrous ridge; dura around confluence of superior petrosal, transverse, and sigmoid sinuses; dura of lateral cerebellar fossa and parietooccipital convexity
ㅤ	Accessory meningeal artery	—	Middle fossa dura, lateral wall of cavernous sinus
ICA	Cavernous carotid	Recurrent artery of foramen lacerum	Dura of foramen lacerum and carotid canal
ㅤ	ㅤ	Medial tentorial artery (Bernasconi & Cassinari)	Roof of cavernous sinus, medial third of tentorium, posterior attachment of falx cerebri
ㅤ	ㅤ	Lateral tentorial artery	Lateral third of tentorium
ㅤ	ㅤ	Dorsal meningeal artery	Dura over dorsum sellae and clivus
ㅤ	ㅤ	Inferior hypophyseal artery	Dura over posterior sellar floor, posterior clinoid, medial wall of cavernous sinus
ㅤ	ㅤ	Medial clival artery	Dura over posterior clinoid, dorsum sellae, medial wall of cavernous sinus
ㅤ	ㅤ	Inferolateral trunk	Inferolateral wall of cavernous sinus, adjacent middle fossa
ㅤ	ㅤ	Capsular arteries	Dura of floor and anterior margin of roof of sella
ㅤ	Ophthalmic artery	Anterior ethmoidal artery	Dura over anterior convexity, medial third of floor of anterior fossa
ㅤ	ㅤ	Posterior ethmoidal artery	Dura over medial third of floor of anterior fossa
ㅤ	ㅤ	Deep recurrent ophthalmic artery	Dura of walls of cavernous sinus around SOF
ㅤ	ㅤ	Superficial recurrent ophthalmic artery	Dura over anterior clinoid process and lesser sphenoid wings
ㅤ	ㅤ	Lacrimal artery	Dura over lateral part of SOF, sphenoid wings, and ridge
ㅤ	Anterior cerebral artery	Olfactory branches	Dura over medial third of floor of anterior fossa
ㅤ	ㅤ	Pericallosal branches	Free edge of falx cerebri
VBA	Vertebral artery	Anterior meningeal artery	Dura over odontoid process, atlantooccipital space, anterolateral border of foramen magnum, including occipital condyles
ㅤ	ㅤ	Posterior meningeal artery	Dura over posterior atlantooccipital space, falx cerebelli, medial and paramedial cerebellar fossa, occipital convexity; walls of transverse sinus and torcula
ㅤ	Anteroinferior cerebellar artery	Subarcuate artery	Dura of posterior surface of petrous bone
ㅤ	Posterior cerebral artery	Tentorial branch	Posterior third of falx cerebri and adjacent tentorium

ECA, external carotid artery; ICA, internal carotid artery; SOF, superior orbital fissure; VBA, vertebrobasilar arteries.

External carotid supply

Anterior cranial fossa	Meningeal branches of ophthalmic artery
Middle cranial fossa	Middle meningeal artery
Posterior cranial fossa	Occipital artery Meningeal branch of ascending pharyngeal artery

Internal carotid (Meningo-pial) supply

Meningo-hypophyseal trunk	Artery of bernasconi and cassinari (see artery)
ACA	Olfactory branches and pericallosal branches Anterior falcine artery
PCA	The artery of Davidoff and Schechter
SCA	Medial dural tentorial branch
AICA	Subarcuate artery
PICA	Posterior meningeal artery

Dural Innervation

Trigeminal nerve	Ophthalmic nerve	Innervation of dura over cribriform plate, medial orbital roof, crista galli, diaphragma sellae, tentorium, falx cerebri, inferior/superior sagittal sinuses, transverse sinus, straight sinus
ㅤ	Maxillary nerve	Innervation of dura of anterior floor of the middle cranial fossa
ㅤ	Mandibular nerve	Innervation of dura over lateral floor of middle cranial fossa and most of the cranial convexity
Upper three cervical roots	Ascending meningeal rami	Innervation of dura lining anterior floor of posterior fossa, clivus, ventral craniospinal junction
ㅤ	Recurrent meningeal branch of vagus	Innervation of walls of sigmoid and occipital sinuses, falx cerebelli, dura over petrous surface of temporal bone, suboccipital cerebellar surface
ㅤ	Hypoglossal nerve	Innervation of dura of posterior fossa extending up to inferior petrosal sinus