Arachnoid Membrane

The arachnoid membrane is the cobweb-like middle layer of the meninges. It lies below the dura mater and over the pia mater. It is an avascular layer (without blood vessels) which is encased in a thin capsule of connective tissue cells and fibers. It has anastomosing connective tissue trabeculae, which project medially from the arachnoid proper towards the surface of the brain and spinal cord, reaching the pia. Therefore, the arachnoid is made up of a membranous roof and irregular trabeculae that look like pillars holding up this roof.

Both of these loose connective tissue components of the arachnoid membrane contain collagen fibers and a few elastic fibers. The flat cells that form the roof are interconnected by cell junctions of all three types, and they are sometimes described as the arachnoid barrier cells. The trabecular fibroblasts are more loosely arranged and have irregular branching processes. The space between the pia mater and the arachnoid is known as the subarachnoid space, which is filled with cerebrospinal fluid. The pia and the arachnoid are considered by some authors as one layer, which is called the leptomeninges.

Although the arachnoid does not follow the uneven surface of the cerebral cortex, like the pia mater does, it does extend across depressions, fissures, and sulci. Numerous thin thread of connective tissue attaches the pia to the arachnoid, thus spanning the subarachnoid space. The subarachnoid space is continuous around the whole central nervous system. Therefore, substances released into the subarachnoid space at one place quickly spread out.

Above, schematic image of the meninges around the brain shows the arachnoid and the other layers of the meninges.