Dura mater

At major boundaries of brain regions such as the longitudinal fissure between the hemispheres, and the tentorium cerebelli between the posterior brain and the cerebellum the dura separates, folds and invaginates to make the divisions. These folds are known as dural folds, or reflections.

The dura mater is primarily derived from neural crest cells, with postnatal contributions from the paraxial mesoderm.

Structure

The dura mater has several functions and layers. The dura mater is a membrane that envelops the arachnoid mater. It surrounds and supports the dural venous sinuses that reabsorbs cerebrospinal fluid and carries the cerebral venous return, back toward the heart.

Cranial dura mater has two layers which include a superficial periosteal layer that is actually the inner periosteum of the neurocranium (the calvaria and endocranium); and a deep meningeal layer, which is the true dura mater. The dura mater covering the spinal cord is known as the dural sac or thecal sac, and only has one layer (the meningeal layer) unlike cranial dura mater. The potential space between these two layers is known as the epidural space, which can accumulate blood in the case of traumatic laceration to the meningeal arteries.

Folds and reflections

The dura separates into two layers at dural reflections (also known as dural folds), places where the inner dural layer is reflected as sheet-like protrusions into the cranial cavity. There are two main dural reflections:

• The tentorium cerebelli exists between and separates the cerebellum and brainstem from the occipital lobes of the cerebrum.
• The falx cerebri, which separates the two hemispheres of the brain, is located in the longitudinal cerebral fissure between the hemispheres.

Two other dural infoldings are the cerebellar falx and the sellar diaphragm:

• The cerebellar falx (falx cerebelli) is a vertical dural infolding that lies inferior to the cerebellar tentorium in the posterior part of the posterior cranial fossa. It partially separates the cerebellar hemispheres.
• The sellar diaphragm is the smallest dural infolding and is a circular sheet of dura that is suspended between the clinoid processes, forming a partial roof over the hypophysial fossa. The sellar diaphragm covers the pituitary gland in this fossa and has an aperture for passage of the infundibulum (pituitary stalk) and hypophysial veins.

Blood supply

This depends upon the area of the cranial cavity:

• In the anterior cranial fossa, the anterior meningeal artery (branching from the anterior ethmoidal artery) is responsible for blood supply
• In the middle cranial fossa, the middle meningeal artery and some accessory arteries are responsible for blood supply. The middle meningeal artery is a direct branch from the maxillary artery and enters the cranial cavity through the foramen spinosum. It then divides into anterior (which runs usually in vertical direction across the pterion) and posterior (which runs posterosuperiorly) branches. The accessory meningeal arteries (which are branches from the maxillary artery) enter the skull through foramen ovale and supply the area between the two foramina
• In the posterior cranial fossa, four different arteries are responsible for the blood supply to the dura mater:

1. posterior meningeal artery (from the ascending pharyngeal artery through the jugular foramen)
2. meningeal arteries (from the ascending pharyngeal artery through hypoglossal canal)
3. meningeal arteries (from the occipital artery through jugular or mastoid foramen)
4. meningeal arteries (from the vertebral artery through foramen magnum)

Drainage

The two layers of dura mater run together throughout most of the skull. Where they separate, the gap between them is called a dural venous sinus. These sinuses drain blood and cerebrospinal fluid (CSF) from the brain and empty into the internal jugular vein.

Arachnoid villi, which are outgrowths of the arachnoid mater (the middle meningeal layer), extend into the dural venous sinuses to drain CSF. These villi act as one-way valves. Meningeal veins, which course through the dura mater, and bridging veins, which drain the underlying neural tissue and puncture the dura mater, empty into these dural sinuses. A rupture of a bridging vein causes a subdural hematoma.

Nerve supply

The supratentorial dura mater membrane is supplied by small meningeal branches of the trigeminal nerve (V1, V2 and V3). The innervation for the infratentorial dura mater are via upper cervical nerves and the meningeal branch of the vagus nerve.