Overview

The arachnoid follows the inner surface of the dura and bridges over the sulci without dipping into them, unlike the pia. Between the arachnoid and pia this creates the subarachnoid space, which is filled with CSF and crossed by delicate collagenous arachnoid trabeculae, giving a spider-web (arachne) appearance. Its outer arachnoid barrier cell layer is joined by tight junctions and forms part of the blood–CSF barrier, separating the CSF from the fenestrated vessels of the dura.

Normally the arachnoid lies directly against the dura with no real space between them. However, the innermost dural border cells have few junctions and little collagen, so this plane splits easily and blood can collect there as a "subdural" haematoma. At intervals the arachnoid herniates through the meningeal dura into the venous sinuses as arachnoid villi and granulations, the main site of CSF absorption. Arachnoid cap cells at the tips of the granulations give rise to meningiomas. In the spinal canal the arachnoid lines the dural sac down to S2, enclosing the lumbar cistern.

Location: Between the dura (outside) and pia (inside) around the brain and spinal cord.

Parts & subdivisions

Arachnoid barrier cell layer
Tight-junction-sealed outer layer; part of the blood–CSF barrier.
Arachnoid trabeculae
Collagen strands crossing the subarachnoid space to the pia; support the vessels.
Arachnoid granulations
Projections into the dural venous sinuses for CSF absorption.

Anatomy

Histology: Avascular; flattened fibroblast-like arachnoid cells with tight junctions on the outer surface; trabeculae of collagen covered by flattened cells.

Function

Encloses the CSF-filled subarachnoid space
Barrier between the CSF and the dura and its blood
Absorbs CSF into the venous system via granulations
Supports cerebral vessels in the subarachnoid space

Clinical significance

Subdural haematoma

Bridging veins pass from the cortex through the subarachnoid space and arachnoid to reach the dural sinuses. They tear with acceleration–deceleration, and blood collects within the dural border cell layer. This happens easily in the elderly, in whom atrophy stretches the veins.

Arachnoid cysts and arachnoiditis

Arachnoid cysts are benign CSF-filled duplications of the arachnoid, commonly in the middle cranial fossa, and are usually incidental. Adhesive arachnoiditis is scarring after infection, haemorrhage, surgery or intrathecal agents. It clumps the nerve roots and causes chronic pain and neurological deficit.

Test yourself

Where is CSF chiefly returned to the venous blood?

Choroid plexus capillaries
Internal vertebral venous plexus
Ependymal cells of the central canal
Arachnoid granulations into the dural venous sinuses

Answer: Arachnoid granulations into the dural venous sinuses

Arachnoid granulations, especially along the superior sagittal sinus, act as pressure-dependent, one-way valves into the dural venous sinuses. Some CSF also drains along cranial and spinal nerve sheaths to lymphatics. Scarring of the granulations after meningitis or SAH causes communicating hydrocephalus.