Arachnoid mater

The arachnoid mater is the middle layer of the meninges, the three membranes that surround and protect the brain and spinal cord. It lies between the outer dura mater and the inner pia mater and is separated from the latter by the cerebrospinal fluid-filled subarachnoid space. The arachnoid is a thin, avascular membrane involved in cushioning the CNS, facilitating CSF circulation, and contributing to CSF absorption through arachnoid villi.

Structure

• Appearance: Thin, translucent, and web-like membrane (hence “arachnoid,” meaning spider-like)
• Composition: Non-vascular connective tissue made of flat, tightly packed cells connected by tight junctions
• Attachments: Does not follow the brain’s surface contours or sulci, unlike the pia mater; loosely connected to the dura above and suspended over the pia below via arachnoid trabeculae

Position and Layers

The arachnoid mater lies immediately beneath the dura mater and above the pia mater. It forms a barrier between the dura and the subarachnoid space and is connected to the pia mater by fine, fibrous trabeculae that span the subarachnoid space.

Subarachnoid Space

The subarachnoid space lies between the arachnoid and pia mater and contains cerebrospinal fluid (CSF), arteries, veins, cranial nerves, and arachnoid trabeculae. This space is continuous around the brain and spinal cord and is enlarged in certain areas to form cisterns (e.g., cisterna magna, interpeduncular cistern).

Arachnoid Villi and Granulations

• Arachnoid villi: Microscopic protrusions of arachnoid mater into the dural venous sinuses, particularly the superior sagittal sinus
• Arachnoid granulations: Macroscopic aggregations of villi visible on the inner surface of the dura; primary sites for CSF absorption into the venous system
• Function: One-way valves that allow CSF to drain from the subarachnoid space into venous blood, driven by pressure gradients

Spinal Arachnoid Mater

In the spinal cord, the arachnoid mater forms a delicate sheath enclosing the spinal cord within the dural sac. It extends from the foramen magnum to about the level of S2 vertebra and is separated from the pia by the spinal subarachnoid space, which contains CSF and is widest in the lumbar cistern.

Function

• Protective cushion: Suspends the brain and spinal cord within the CSF-filled subarachnoid space
• CSF dynamics: Permits circulation of CSF around the CNS and absorption into venous sinuses
• Barrier role: Forms part of the blood-CSF barrier; limits substance exchange between dura and CSF

Blood Supply

The arachnoid mater is avascular and does not have a direct blood supply. It receives nutrients via diffusion from CSF and nearby vascular structures in the pia or dura mater.

Innervation

The arachnoid mater has minimal or no sensory innervation. Pain associated with meningeal irritation (e.g., in meningitis) primarily arises from the richly innervated dura mater.

Development

The arachnoid and pia mater develop together from the neural crest as a single layer called the leptomeninges. They later differentiate into two distinct layers. The subarachnoid space forms as a cleft between these layers and becomes filled with CSF during fetal development.

Clinical Significance

• Subarachnoid hemorrhage: Bleeding into the subarachnoid space, typically due to aneurysm rupture; presents with severe headache, neck stiffness, and photophobia
• Arachnoid cysts: Congenital CSF-filled sacs located between the arachnoid and pia; may be asymptomatic or cause mass effect
• Arachnoiditis: Inflammatory condition of the arachnoid mater; often due to surgery, infection, or trauma; can cause chronic pain and neurological deficits
• CSF flow obstruction: Adhesions in the subarachnoid space (e.g., from meningitis) may impair CSF circulation and lead to hydrocephalus

Imaging

• MRI: Visualizes arachnoid cysts, thickening, or enhancement (e.g., in arachnoiditis); FLAIR and contrast-enhanced sequences are particularly useful
• CT scan: May detect large cysts or calcifications; less sensitive for fine arachnoid pathology
• Myelography: May demonstrate CSF flow obstruction or arachnoid adhesions in spinal disorders