Meninges and their importance

Brain meninges are three-layer tissue envelopes that have a protective, supportive and metabolic role. They are located between the brain and the skull and between the spinal cord and spinal vertebrae and are constructed of loose and dense connective tissues.

These are three membranes that
surround the brain and spinal cord: the outer hard membrane (dura mater), the
soft membrane (pia mater) which is rich in blood vessels, and the internal
connective membrane (arachnoidea mater) that keeps the brain immersed in the
cerebrospinal fluid (1).

Dura mater

The most external membrane is dura mater. It is made up of solid connective tissue and completely encloses the brain and spinal cord. It consists of two sheets:

• The outer, periosteal layer lining
  the inner side of the bones of the skull and spinal canal, richly supplied with
  blood vessels and nerves,
• The inner, meningeal layer whose
  interior is covered with a monolayer mesenchymal epithelium (2).

The meningeal layer is connected to the “spider web” of the Arachnoidea from which it is separated by the spatium subdurale. The spatium epidurale s. extradurale is located between the two layers of the dura mater.

It is especially pronounced in the medullae spinalis region where it is filled with adipose tissue and plexus venosus vertebralis internus anterior and posterior. In the skull, the epidural space is virtual and exists only in the area of the large vessels of the dura mater. 

After removal of brain masses from the skull, thick partitions are observed, which build up duplications of the dura mater, and are formed by protrusion of the meningeal layer of hard membrane into the cranial cavity. These compartments separate the individual parts of the brain, support them and thus ensure their permanent position during various movements and head positions.

Some barriers are placed sagittally (falx cerebri and falx cerebelli) and some transversely (tentorium cerebelli, cavum trigeminals, and diaphragma sellae).

The falcous septum (falx cerebri) is a duplication of the dura mater located between the cerebral hemispheres, inserted into the fissura interhemispherica. The anterior part of the falx attaches anteriorly to the crista gali ossis ethmoidalis.

Its upper edge is convex and goes backwards along the inside of the roof of the skull to the segment called protuberantia occipitalis internal. The upper edge splits and catches on the edges of the sulcus sinus sagittalis superioris, thus forming the eponymous venous sinus, the sinus sagittalis superior.

The lower edge of the falx of the cerebri is concave and free. It is located along the upper, convex side of the corpus callosum and contains the sinus sagittalis inferior. Its base is located at the back and positioned low, and connects in the medial line with the tentorium cerebelli.

The falcous septum (falx cerebelli) is a mediosagittally located duplication of the dura mater inserted between the cerebral hemispheres. It is located below the tentorium cerebelli. The posterior margin of this septum is convex and attached to crista occipitalis internally, and the anterior margin is free and concave.

The tentorium cerebelli is a duplicate dura mater element located in the transverse plane. It is positioned directly below the cerebral lobes of the cerebellum, and divides the cranial cavity into two parts: the upper part (cerebral osteo-fibrotic lobe), which houses the telencephalon and diencephalon, and the lower part (cerebellar osteo-fibrotic lobe).

The posterior edge of the tentorium is convex and binds to the edges of the sulcus sinus transversi, thus encompassing the sinus transversus. At the point where the sulcus sinus transversi crosses into the sulcus sinus sigmoidei, the edges of the tentorium approach the upper edge of the pyramids of the temporal bones.

In some pathological cases, resulting in an increase of the intracranial pressure (intracranial hypertension), sharp edges of the incisurae tentorii may occur on the mesencephalon or entrapment of the cinguli gyrus (transtentorial or uncalated incantation), leading to severe neurological complications, sometimes with a lethal outcome.

The narrow space between the mesencephalons and the edges of the tentorium is the only communication between the subtentorial and supratentorial areas of the subarachnoidal space.

The cerebrospinal cortex circulates slowly around the mesencephalon upwards to the upper edge of the cerebri falx where it is absorbed into the venous blood by the sinus sagittalis superior via the granulationes arachnoideae.

Diaphragma sellae is a duplication
of the dura mater interspersed between the processus clinoidei anteriores and the
tuberculum sellae, anterior and upper edges of the dorsum sellae and the
processus clinoidei posteriores. The pituitary gland is located below the
diaphragma sellae in the fossi hypophysialis of the cuneiform bone. In the
middle of the diaphragmae sellae there is an opening through which the
infundibulum passes, on which the neurohypophysis is attached.

The trigeminal cavity (cavitas
trigeminalis) is a duplication of a hard brain membrane located in the
impressio trigeminalis area at the anterior face of the pyramid of the temporal
bone. It has the shape of a three-finger glove containing the roots of nervus
trigeminus (radix motoria and sensitiva), ganglion trigeminals and the initial
parts of the three branches of the nervus trigeminus (those are the nervus
ophtalmicus, nervus maxillaris and nervus mandibularis).

Arachnoidea Mater

The arachnoidea mater is a thin elastic membrane made up of collagen and elastic connective strands lined on both sides by the endothelium. Together with the pia mater it forms a soft membrane or leptomeninx.

It is characterized by the absence of blood vessels and nerves. It is separated from the durae mater by a subdural space (spatium subdurale). Below the arachnodeae mater there is a subarachnoid space filled with cerebral fluid (liquor cerebrospinalis) (3).

Numerous gentle connective trabeculae pass through the subarachnoid space, connecting the arachnoid and the pia mater.

In the convex sites, i.e. in the area of the cerebral curvature, these two membranes have partly grown together, while in the area of cerebral furrows and cracks they diverge, because the pia mater sinks into the furrows and fissures, and arachnoidea bridges them like a spider's web, hence its name (often called “spider web”).

In the subarachnoid space, there are the blood vessels of the brain (circulus arteriosus cerebri and its branches) and the initial parts of all nerve.

Pia mater

The pia mater is an internal brain membrane located directly adjacent to the surface of the brain and spinal cord. It represents a visceral layer of leptomeninx. It follows all the external forms of the brain and spinal cord, with which it is closely connected. It is made up of collagen binders and is richly vascularized (4).

On the cerebellum, pia mater
encephali coats all the gyri cerebri and enters the brain furrows (sulci
cerebri). In the cerebellum, the pia mater covers its surface, sinking only
into deeper fissures (fissurae cerebelli), while sending internal extensions
that only partially enclose the cerebellar furrows (cerebellar sulci) from the
inner surface.

The soft membrane of the spinal cord (pia mater spinalis) extends from the occipital opening to the second lumbar vertebra. From the outside, it is in contact with the cerebrospinal fluid through which pass the gentle trabeculae that connect the pia mater to the arachnoide.

At the same time, the pia mater is connected to the dura mater via arachnoideae by the so-called “toothed connections”, scientifically called the ligg. denticulata. These connections are triangular in shape with a base attaching to the pia mater laterally to the spinal cord, between the anterior and posterior roots of the spinal nerves, and the tip terminating on the inner layer of the dura mater.

Meningeoms - Tumors of the Brain Meninges

These tumors grow from the middle layer of the brain membranes, i.e. the arachnoidea mater. Meningomas represent the most common type of primary brain tumors, accounting for about 30% of all brain tumors and 25% of adult brain tumors, while less commonly affecting children (2%). They occur most commonly in elderly patients, with the highest incidence between the ages of 70 and 80 and are 3 times more common in women than in men.

They are mostly intracranial, most
commonly localized in the forebrain and cerebellum, while about 10% of them are
spinal.

They can be slow-growing-low-grade
or fast-growing-high grade tumors. They are mostly benign (grade I) but some
are atypical. Those of Grade III are aggressive and anaplastic.

Conclusion

The brain membranes or meninges
surround and protect the brain. There are three different meninges. Those are
the dura mater, pia mater, and arachnoidea mater. The soft envelope, or pia, is
located directly on the glia. The arachnoidea mater is located on top of it,
and the exterior, hard envelope or dura mater is located above the arachnoidea.

The pia mater is the inner lining that follows the shape of the brain, enveloping all the folds, protrusions and furrows. It completely envelops the nerve roots. The arachnoidea mater is located in between the three envelopes and is coated with the endothelium.

It is separated from the hard sheath by the subdurale cavum that is filled with liquor. The hard cover is on the outside. It is a rigid fibrous sack whose task is to cover the brain for its protection. It is very rigid and of different thicknesses.

References

1. Purves D, Augustine GJ, Fitzpatrick D, et al., editors. Neuroscience. 2nd edition. Sunderland (MA): Sinauer Associates; 2001. The Meninges.  Found online at: https://www.ncbi.nlm.nih.gov/books/NBK10877/
2. Marina Protasoni et al. The collagenic architecture of human dura mater. J Neurosurg. 2011 Jun; 114(6): 1723–1730. Published online 2011 Feb 4. doi: 10.3171/2010.12.JNS101732 Found online at: https://www.ncbi.nlm.nih.gov/pubmed/21294622
3. Vandenabeele F, Creemers J, Lambrichts I. Ultrastructure of the human spinal arachnoid mater and dura mater. J Anat. 1996 Oct;189 ( Pt 2)(Pt 2):417-30. PMID: 8886963; PMCID: PMC1167758.  Found online at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1167758/
4. Adeeb N. et al. The pia mater: a comprehensive review of literature. Childs Nerv Syst. 2013 Oct;29(10):1803-10. doi: 10.1007/s00381-013-2044-5. Epub 2013 Feb 5. Found online at: https://www.ncbi.nlm.nih.gov/pubmed/23381008