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Arachnoid mater

Definition, location and structure of the arachnoid mater.

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Hello everyone! This is Matt from Kenhub, and in this tutorial, we will be looking at the middle layer of the meninges – the arachnoid mater. To begin, let's first look at the meninges. The meninges is the collective term for the connective tissues that cover the brain and spinal cord. Together, they line the cranium and vertebral canal providing protection to the central nervous system, the CNS.

The three layers of tissues that form this protective covering are shown here from outermost to innermost layer - the dura mater, the arachnoid mater and the pia mater. The arachnoid mater is the middle layer of the meninges inferior to the dura mater but superior to the pia mater. The term arachnoid is a reference to the spider web-like appearance of the arachnoid mater fibers where they attach to the underlying pia mater. The arachnoid mater is a delicate, impermeable, avascular membrane reinforced by the inner meningeal layer of the dura mater. Highlighted in green in the images, it covers the brain and the spinal cord in a similar arrangement and contributes to the circulation of cerebrospinal fluid (CSF) throughout the central nervous system.

A potential space called the subdural space exists between the inner layer of the dura mater and the tightly aligned arachnoid mater. The subdural space can develop when hemorrhage-inducing brain trauma results in the build-up of blood between these two layers. This is called a subdural hematoma. Another space formed by the arachnoid mater is the subarachnoid space indicated by the green highlighted area in this section of spinal arachnoid mater. It is located between the arachnoid mater and the underlying pia mater and contains the arachnoid trabeculae – thin fibrous filaments that hold the two layers in place.

The subarachnoid space bounded by the impermeable arachnoid and pia mater is filled with CSF and provides a pathway for CSF circulation and absorption around the brain and spinal cord. This space extends down to the termination of the spinal arachnoid mater at the level of the S2 vertebra. It is also important to note that cranial nerves, roots of spinal nerves, arteries and veins from both the brain and spinal cord pass through the subarachnoid space.

Cranial and spinal arachnoid mater are very similar in structure and function, however, cranial arachnoid mater has certain features that are mostly unique to itself. Subarachnoid cisterns are notable feature of cranial arachnoid mater. These are spaces within the subarachnoid space where cerebrospinal fluid pools and many vessels and nerves exit toward skull foramina. This pooling is a result of the cranial pia mater being tightly adhered to every fissure and contour of the brain while the cranial arachnoid mater like the cranial dura mater more loosely envelops just the immediate surface of brain. This difference enables large gaps to exist between the cranial pia and arachnoid mater in areas where the cranial pia mater descends into a sulcus.

An exception to cistern formation within the cranial subarachnoid spaces is the lumbar cistern, located at the termination of the spinal canal extending from vertebral level L1 or L2 down to the S2 sacral foraminal level. The lumbar cistern is contained within the dural sac of the spinal cord and is where CSF is drawn from during a lumbar procedure.

Another unique feature of the cranial arachnoid mater are arachnoid villi. In certain areas, the cranial arachnoid mater herniates through small openings in the overlying cranial dura mater into the dural venous sinuses. These herniations are called arachnoid villi. The villi allows CSF to be reabsorbed by the brain's venous blood within the dural venous sinuses. They are most commonly found in the superior sagittal sinus shown here in green and its associated blood lakes also called lateral lacunae now highlighted in green. As humans age, arachnoid villi become aggregated into large clumps called arachnoid granulations which can be seen highlighted in green in the image. These clumps are also primarily located within the superior sagittal sinus.

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