Arachnoid granulations

Arachnoid Granulations (also known as Arachnoid Villi) are small protrusions of the arachnoid mater (the middle layer of the meninges) into the outer membrane of the dura mater. They are responsible for the transfer of cerebrospinal fluid (CSF) from the subarachnoid space to the venous system.

Structure

Arachnoid granulations are composed of a cluster of arachnoid cells. They are typically found in the superior sagittal sinus, one of the dural venous sinuses, but can also be found in other dural sinuses and in the spinal canal. The size of arachnoid granulations can vary, with larger granulations often referred to as Pacchionian granulations.

Function

The primary function of arachnoid granulations is the reabsorption of CSF from the subarachnoid space into the venous system. This process helps to maintain a constant pressure within the central nervous system (CNS). The CSF passes through the arachnoid granulations by a process of bulk flow, rather than by active transport.

Clinical Significance

Arachnoid granulations can become enlarged or hypertrophic, often in response to increased intracranial pressure. This can lead to the formation of arachnoid cysts or meningoceles. In rare cases, arachnoid granulations can erode into the inner table of the skull, causing a condition known as Granulation pits.

See Also

• Meninges
• Cerebrospinal fluid
• Central nervous system