From the outside to the inside, the dura mater, arachnoid mater, and pia mater can be distinguished. The arachnoid and pia mater form the leptomeninges (soft meninx) and originate from one single layer. Above them is the dura mater (hard meninx), which is also referred to as pachymeninx. The meninges, in combination with the subarachnoid space, which is filled with cerebrospinal fluid, and the skull, protect the brain from trauma.
Structure of the dura mater
The dura mater consists of firm, collagenous connective tissue organized in two layers:
- The superficial layer is known as the periosteal layer. This layer serves as the inner layer of the skull (endocranium).
- The inner meningeal layer is considered the true dura mater. When the dura mater covers the spinal cord, it is known as the thecal sac.
An exception is in places in which the emissary veins run through the dura mater (see figure). The above-mentioned veins then flow into the dural venous sinuses which are located between the two layers of the dura mater.
Apart from the emissary veins, the bridging veins which run between the arachnoid mater and the meningeal layer of the dura mater flow into the dural venous sinuses. Injury to these bridging veins results in subdural bleeding. Because of the tight connection between the dura and arachnoid mater, a true subdural space does not usually exist. However, subdural bleeding may cause the neurothelium of the arachnoid mater (see below) to detach from the dura mater, creating an artificial subdural space.
Septa of the dura mater
In some places, the dura mater extends into the brain and forms septa or duplications of the meningeal layer of the dura. Four septa of the dura mater can be identified.
Falx cerebri/cerebral falx: This is the largest septum which is sickle-shaped and runs through the longitudinal (interhemispheric) fissure, separating the two brain hemispheres. The falx cerebri originates in the crista galli of the ethmoid bone. It separates the two cerebral hemispheres.
Falx cerebelli/cerebellar falx: This is a vertical dural fold that lies inferior to the cerebellar tentorium in the posterior cranial fossa separating the cerebellar hemispheres.
Tentorium cerebelli/cerebellar tentorium: This separates the occipital lobe of the brain from the cerebellum. Further, the cerebellar tentorium can help one differentiate between the supratentorial telencephalon and the infratentorial cerebellum. It has an opening, the tentorial incisura, for the brain stem to pass through.
Diaphragma sellae: Covers the sella turcica and has a small opening for the pituitary stalk and hypophyseal veins to pass through.
Blood supply of the dura mater
The blood supply of the dura mater, along with the supply of the periosteum and the neighboring cranial bones, comes from three arteries. The main supply comes from the middle meningeal artery, which is a branch of the maxillary artery. The two other arteries, the anterior and posterior meningeal arteries, play a minor role from a clinical point of view.
In traumatic brain injury, the middle meningeal artery may tear, resulting in epidural bleeding and creating the epidural space which normally does not exist because of the tight connection between the dura mater and the periosteum.
Innervation of the dura mater
The dura mater is innervated by the trigeminal nerve (cranial nerve V), glossopharyngeal nerve (cranial nerve IX), and vagus nerve (cranial nerve X), as well as via the first two branches of the cervical nerves. In meningitis, these sensitive nerves are irritated, resulting in headaches and reflective stiffness of the neck. To relax the meninges as much as possible, the head is held in a certain way which is supposed to be relaxing. However, this leads to a hyperextension of the head. Unlike the dura mater, the brain has no sensory innervation.
Diseases of the dura mater
The dura mater is composed of firm collagenous connective tissue; thus, space-occupying conditions such as bleeding or tumors in the cranial fossa may lead to incarcerations in the area of the septa. Here, axial and lateral incarcerations may occur.
Axial incarceration represents symmetrical incarceration due to a process that occurs in both hemispheres (i.e. cerebral edema). In the case of upper axial incarceration, the two middle and lower parts of the temporal lobe are pressed through the slit of the tentorium cerebelli and apply pressure on the midbrain (mesencephalon).
In cases of lower axial incarceration, a displacement of the cerebellar tonsils through the foramen magnum and compression of the brainstem occurs.
Lateral incarceration occurs in unilateral, space-occupying conditions (i.e. brain tumor or bleeding). In this case, a displacement of the cerebral crura to the opposite side takes place due to an ipsilateral herniation of the temporal lobe in the area of the cerebellar tentorium. As a result, the pyramidal tract is damaged before it crosses over to the contralateral side. Here, the muscles of the opposite side are affected.
The arachnoid mater is the middle layer of the meninges and consists of a fine layer of connective tissue. The part of the arachnoid mater that borders the dura mater comprises multiple layers of flat cells (meningeal cells). These layers are defined as the neurothelium since the membrane is a derivative of the neural crest mesectoderm of the embryo. There, the meningeal cells are closely connected via tight junctions and form a barrier between the dura mater and the subarachnoid space (blood-cerebrospinal fluid barrier).
The arachnoid mater is closely associated with the dura mater superiorly and the inner surface covers the brain (arachnoidea encephali) and spinal cord (arachnoidea spinalis). The part covering the brain cavity does not form folds like the dura mater apart from the region of the longitudinal fissure. The part covering the spinal canal continues until the level of S2 where it ends as the filum terminale attached to the coccygeal end of the spinal column.
The border of the subarachnoid space is formed by the outer arachnoid mater and the inner pia mater. Within the subarachnoid space, there are numerous connective tissue trabeculae, which are also covered by meningeal cells.
The subarachnoid space contains the cerebrospinal fluid, which is produced in the area of the choroid plexus (roof of the 3rd and 4th ventricles and the wall of the lateral ventricles) and forms the outer cerebrospinal fluid space.
The cerebrospinal fluid finds its way through three openings in the roof of the 4th ventricle into the subarachnoid space, and from there via the arachnoid villi, into the venous vascular system. The arachnoid villi are bulges of the arachnoid mater, with parts of the subarachnoid space in the lumen of the dural venous sinuses. They serve to reabsorb the cerebrospinal fluid into the blood.
The subarachnoid cisterns are compartments within the subarachnoid space where the pia and arachnoid mater are not in close contact, and cerebrospinal fluid pools in these compartments.
The most important cistern is the cerebellomedullary cistern, which is located between the cerebellum and the medulla oblongata. This cistern may be used via a suboccipital puncture to extract cerebrospinal fluid for clinical diagnostics.
However, in the daily clinical routine, the lumbar cistern, located in the lower lumbar area, is used far more often to obtain cerebrospinal fluid (lumbar puncture) as there are fewer complications compared with a suboccipital puncture. Additional cisterns include the cistern of the lateral cerebral fossa, the ambient cistern, and the interpeduncular cistern.
The pia mater rests directly on the gyri of the brain and continues into the respective sulci. The Virchow-Robin space, which is located between the cranial pia mater and the external limiting membrane of the brain, separates the pia mater from the brain. The cranial pia mater consists of a thin layer of connective tissue cells (meningeal cells) which brings about a shimmering appearance to the brain’s surface.
The cranial pia mater is very vessel-rich and forms the outermost layer of blood vessels that enter or exit the CNS and eventually enters into the CNS along with them. Together with the arachnoid mater, the pia mater forms the border of the subarachnoid space (see above).
The arachnoid mater, the cranial pia mater, and the subarachnoid space continue as the spinal arachnoid mater and pia mater in the vertebral canal.