Dura Mater

Overview:

The brain is an important part of the human body, as it performs vital functions and controls almost all bodily functions. It is protected by a strong framework of bones, called the skull. Besides this bony structure, it is also protected against several kinds of injuries or head traumas by the meninges, which are three-layered membranous structures that help protect the brain parts from damage.

The outermost layer of the meninges is the dura mater. The other two layers are collectively known as the leptomeninges and consist of the arachnoid mater (middle layer) and the pia mater (innermost layer). The dura mater is frequently known as the dura.

The dura protects its vital underlying components with a strong fibrous protective covering.  In aspects of compressive impacts due to mass lesions and related edema, the compartmentalization of the brain through the help of dural reflections is especially important. The connection of the dural layers to each other, and the underlying leptomeninges and calvarium, has a huge impact on scientists’ knowledge of the impacts of intracranial injury.

In this article, the location, structural components, function, and clinical complications involving the dura mater are discussed.

Summary:

•   The meninges safeguard the spinal cord and brain from tissue injury and assist the framework of the blood vessels
• Dura mater is on the outermost end of the meninges, situated directly beneath the skull and the bones of the vertebral column
• The dura mater is made up of two layers of connective tissue: the periosteal layer and the meningeal layer
• The periosteal layer lines the inner surface of the cranium’s bones. The periosteum is a dense fibrous membrane that covers the surface areas of bones
• The meningeal layer is found deep within the periosteal layer
• The dura mater has a vascular supply of its own
• The dura mater’s meningeal layer bends towards the inner direction of itself to form 4 structural forms, called the dural reflections
• The 4 dural reflections are: Diaphagma sellae,  Falx cerebri, Tentorium cerebelli, and Falx cerebelli
• Dura mater encircles and continues to support the large venous channels (called dural sinuses) that help in carrying the blood from the brain to the heart.
• Haemorrhage, meningitis, and meningiomas are some complications involving the dura mater

The Meninges:

The brain is a mass of neural tissue located in the anterior end of a living thing. The brain incorporates sensory input and instructs motor responses. It is also the centre of gaining knowledge in higher vertebrates. The human brain weighs about 1.4 kilograms (1.4 kg or three pounds) and is composed mainly of millions and millions of interconnected neurons. To summarize these points, it can be undoubtedly claimed that the brain is one of the most important organs of the human body, so it only makes sense that it needs to be protected.

Thus, the brain and spinal cord are protected by three (3) layers of membranes which are referred to as the meninges (Figure 1). The meninges safeguard the spinal cord and brain from tissue injury, assist the framework of the blood vessels, and create a complete cavity through which the cerebrospinal fluid (or shortly called the CSF) flows. The CSF travels between the two meningeal layers present on the inner side, which are the arachnoid (middle) layer and the pia (innermost) layer. These two layers are collectively given the name: the leptomeninges. These three layers are:

1. Pia Mater: The pia mater is the sensitive lining present in the innermost part, so it forms the innermost layer. The pia mater lies beneath the subarachnoid area. It is quite thin and firmly attached to the brain and spinal cord’s exterior. The Pia mater is the only layer that outlines the shape of the brain, including the gyri (brain bends and creases that give it a wrinkled impression) and fissures (the deep groove that distinguishes the vertebrate brain’s two cerebral hemispheres). 

2. Arachnoid Mater: The arachnoid is in the middle part, and it is in the form of a web-like framework that is filled with fluid that blankets the brain. The arachnoid mater is the meninges’ middle layer, located directly beneath the dura mater. It is made up of connective tissue layers, is avascular (has few or no blood vessels), and has no innervation (distribution or supply of nerves). 

3. Dura Mater: The dura mater is the tough lining that is present in the outermost part of the meninges, so it forms the outer layer.

Location:

The dura mater is the layer that is present on the outermost end of the meninges, situated directly beneath the skull and the bones of the vertebral column.  It’s thick, resilient, and unable to stretch or be drawn out in length. Structure:

The dura mater is made up of two layers of connective tissue (Figure 2):

Periosteal layer: The periosteal layer lines the inner surface of the cranium’s bones. The periosteum is a dense fibrous membrane that covers the surface areas of bones and is made up of a fibrous coating present on the outermost side and an inner cellular layer called cambium. The layer present at the outer side outer is majorly made of collagen and comprises nerve fibres, which cause pain when the tissue is injured or harmed in any way. It also has numerous blood vessels. The branches of these blood vessels pierce through and enter the bone to supply the osteocytes (commonly called bone cells). 

It should be noted that these branches enter the bone with the help of Volkmann canals and connect to vessels in the Haversian canals, which are present along the length of the bone. There are special kinds of fibres for attaching the bone to the periosteum, these are called Sharpey fibres. Sharpey fibres are formed when fibres from the inner layer pierce through the underlying bone and team up with blood vessels to aid in attaching the periosteum to the bone.

The periosteum’s inner layer comprises osteoblasts, which are the cells responsible for bone production. It is most visible during foetal life and early stages of childhood because this is when the process of formation of bones is at its peak.

Meningeal layer: It is found deep within the periosteal layer. It is connected to the spinal cord’s dura mater.

Except for the region where they differentiate to encapsulate the dural venous sinuses, the two dural layers are strongly bound to each other. The meningeal layer protrudes inward towards the cerebral tissue in such locations, creating the fibrous septa. These fibrous septa are responsible for partially separating the cranial cavity.

The dural venous sinuses are positioned between the dura mater’s two layers. They are in charge of cranial venous drainage and lead into the jugular veins in the internal regions.

The dura mater has a vascular supply of its own.  The middle meningeal vein and artery are the primary suppliers of the dura mater. As for the nerve network, trigeminal nerve innervates it (V1, V2 and V3).

Dural Reflections: The dura mater’s meningeal layer bends towards the inner direction of itself to form 4 structural forms, called the dural reflections.

These four dural reflections protrude into the cavity inside the cranium and there, they divide it into many chambers or compartments. All of these chambers contain a further subdivision of the brain. These four dural reflections are:

1. Diaphagma sellae: covers the sphenoid bone’s hypophysial fossa. It has a tiny hole for helping the pituitary gland stalk to pass through it.

2. Falx cerebri: The falx cerebri is the most extensive of the fibrous septa. It runs from the crista galli to the internal occipital protuberance all across the middle line on the innermost surface of the calvaria. It is a downward-projecting structure that separates the left and right cerebral hemispheres.   It not only separates the left and right cerebral hemispheres but also contains the inferior and superior sagittal sinuses. The falx merges with the tentorium cerebelli posteriorly.

3. Tentorium cerebelli: This helps in distinguishing the occipital and cerebellar lobes which extends in a transverse plane from the occipital bone’s inner surface It connects the cerebrum and the cerebellum and houses the straight, superior and transverse petrosal sinuses. The tentorium splits the intracranial area into supratentorial and infratentorial components. The supratentorial compartment is where the forebrain is present, and the infratentorial compartment is where the hindbrain resides.  The tentorial notch is an area located anteromedially for the passage of the midbrain.

4. Falx cerebelli: the membrane that separates the left and right cerebellar hemispheres.

The meningeal dura mater covers the trigeminal ganglion, enveloping it in the trigeminal cave (also referred to as the Meckel’s cave).

Size and Dimensions:

The complete thickness of the dura mater at the point of the 3rd lumbar vertebrate ranges from seventy-eight to eighty-two (78-82) laminae, measuring two hundred and seventy microns (270 microns) in total. The dura mater is composed of many sublaminae that run concentrically parallel to the medulla’s surface.

Function:

The dura mater is a capsule that surrounds the arachnoid and has been adapted to perform a variety of functions. The dura mater encircles and continues to support the large venous channels (called dural sinuses) that help in carrying the blood from the brain to the heart. The dura mater divides into two separate sheets within the vertebral canal, which is segregated by the epidural region, which is packed with numerous veins. The periosteum of the vertebral canal is made up of the outer surface of these two sheets. The restrictive subdural area (that is fluid-filled), separates the inner sheet from the arachnoid. 

A very significant area of fusion between the two meninges is in the wall surfaces of the dura mater’s huge venous channels, where arachnoid elongations, permeate the dura mater and protrude into the veins. These arachnoid finger-like structures, known as arachnoid villi (or arachnoid granulations), play a role in the passage of cerebrospinal fluid from the subarachnoid space.

Complications involving the dura matter:

1. Epidural haemorrhage: Epidural haemorrhage is an uncommon condition induced by the breaking of a meningeal vessel.  The cause of the bleeding is generally the middle meningeal artery or, in rare circumstances, one of the dural venous sinuses. Epidural bleeding is typically caused by an injury of the parieto-temporal region of the skull, which frequently results in the bursting of the aforementioned blood vessels. The periosteal dura mater is separated from the skull by the haemorrhage, resulting in an epidural haematoma in between the dura mater and the calvaria. 

Epidural bleeding is more common in younger people because the periosteal dura becomes more tightly bound to the diploë with age. A CT scan is used to make a diagnosis for an epidural haematoma, which appears as a biconvex hyper dense structure. The biconvex shape is generally caused by a strong connection of the dura to the cranial sutures, which limits bleeding. If the individual is treated with a surgical procedure and at the appropriate time, he or she will be cured in a few days. 

2. Meningitis: Meningitis is a potentially fatal condition characterized by inflammation of the meninges (Figure 3). Meningitis is usually caused by a cerebrospinal fluid infection. Microbes such as viruses, bacteria, and fungi are all disease-causing agents that can cause meningeal inflammation. Meningitis can cause brain damage, seizures, and death if not allowed for treatment.  Bacteria are the most common reason for infection. Neisseria meningitidis and Streptococcus pneumoniae are the most common microorganisms that are responsible for meningitis. The immune system’s response to the infection results in cerebral oedema, which increases intracranial pressure. This consequently, has two major impacts: cranial herniation and reduced cerebral perfusion. Cranial herniation occurs when a portion of the brain is forced out of the cranial cavity. Elevated intracranial pressure, when combined with systemic hypotension, decreases cerebral perfusion. Both of these complicated health problems can quickly lead to death.

3. Subdural bleeding: The method of gathering blood within the subdural region, between the dura and arachnoid mater, is referred to as subdural haemorrhage (Figure 4). It is commonly caused by a traumatic blow to the head or some other serious head injury and the fracture of the connecting veins that link the superficial veins of the brain to the dural venous sinuses. Subdural hematomas are most commonly found in the convexities of the skull. They are typically diagnosed using a head CT, which shows them as high-density crescent-shaped patterns. Subdural hematomas are classified as acute or chronic based on how long they have been present.

• Acute subdural haematoma: Acute subdural haematomas are most commonly caused by a powerful force that results in the ripping of the bridging veins, has progressive development, and are closely correlated with underlying brain damage. If the tumour is compressing the brain, it is removed through a surgical procedure.  Small asymptomatic incidents, on the other hand, can be handled expectantly.
• Chronic subdural haematoma: Even minor or unimportant head injuries can lead to chronic subdural haematoma. It is most commonly observed in elder people.. Many of its risk factors include alcoholism, seizures, Cerebrospinal fluid shunts, coagulopathies, and patients who are prone to falling (for instance: hemiparesis from a stroke). Patients usually do not recall any distress that may have triggered the subdural bleeding, which typically occurs during the fall. The chronic subdural bleeding and haematoma development are both very slow. The appearance of symptoms takes place only when the haematoma has grown to a critical point and is pushing on the underlying brain tissue. A CT scan is usually used to make a diagnosis of chronic subdural bleeding, which appears as a crescent-shaped development with a fibrous capsule. The treatment is exactly similar to the case of acute subdural haematomas, which is a neurosurgical operation.

4. Meningiomas: Meningiomas are tumours that form within the meninges. They begin in the arachnoid mater and grow in size, putting pressure on the brain and spinal cord. The majority of meningiomas are harmless and they grow at a very slow pace. Nevertheless, some can grow properly and become cancerous. Meningiomas can develop to be quite big, and surgery is frequently required for treatment.

Conclusion:

Dura mater is the outermost layer of the meninges. It has its vascular supply. The dura mater’s meningeal layer bends towards the inner direction of itself to form 4  structural forms, called the dural reflections. These are Diaphagma sellae,  Falx cerebri, Tentorium cerebelli, and Falx cerebelli. Dura mater encircles and continues to support the large venous channels (called dural sinuses) that help in carrying the blood from the brain to the heart. The structure and position of the dura mater make it a very reliable protective envelop, which is one of its important functions.  Haematomas, meningitis, and meningiomas are some of the diseases that involve damage to the dura mater.

Reference list:

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