Arachnoid granulation


Arachnoid granulations are small protrusions of the arachnoid mater into the outer membrane of the dura mater. They protrude into the dural venous sinuses of the brain, and allow cerebrospinal fluid to exit the subarachnoid space and enter the blood stream.
The largest granulations lie along the superior sagittal sinus, a large venous space running from front to back along the center of the head. They are, however, present along other dural sinuses as well.

Function

Diffusion across the arachnoid granulations into the superior sagittal sinus returns CSF to the venous circulation.
The arachnoid granulations act as one-way valves. Normally the pressure of the CSF is higher than that of the venous system, so CSF flows through the villi and granulations into the blood. If the pressure is reversed for some reason, fluid will not pass back into the subarachnoid space. The reason for this is not known. It has been suggested that the endothelial cells of the venous sinus create vacuoles of CSF, which move through the cell and out into the blood.
The importance of arachnoid granulations for the drainage of CSF is controversial. By some accounts, a large portion of CSF is drained through lymphatics associated with extracranial segments of the cranial nerves. A large proportion of CSF is believed to leave the cranial vault through the axons of CN I through their extension through the cribriform plate.
On the inner surface of cranial bones, small pits called granular fovea are produced by arachnoid granulations.

Eponym

Occasionally, they are referred to by their old name: Pacchioni's granulations or pacchionian bodies, named after Italian anatomist Antonio Pacchioni.

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