Arachnoide

Arachnoide, in anatomical usage, refers to the arachnoid mater, the middle of the three meninges that envelop the central nervous system. The term derives from Greek arachne “spider,” reflecting the web-like trabeculae that permeate the subarachnoid space. The arachnoid mater lies between the dura mater (outer) and pia mater (inner) and is a thin, avascular membrane. It is separated from the dura by the dural border cell layer and is connected to the pia by delicate collagenous trabeculae, forming the subarachnoid space through which cerebrospinal fluid circulates. The arachnoid is flexible but resilient, helping to cushion the brain and spinal cord.

Within the subarachnoid space, the arachnoid granulations protrude into venous sinuses (notably the superior sagittal sinus) and serve as one-way routes for CSF drainage into the bloodstream. Developmentally, the arachnoid mater arises primarily from neural crest-derived meninges during embryogenesis.

Clinically, pathology affecting the arachnoid includes arachnoid cysts, arachnoiditis (inflammation of the arachnoid), and involvement in subarachnoid hemorrhage or meningitis, which can disrupt CSF dynamics and brain function.

Variations of the term exist in other languages and in related anatomy, such as arachnoid trabeculae and barrier cells, which collectively contribute to the integrity and function of the central nervous system coverings.