Arachnoid granulation

Arachnoid granulations (also arachnoid villi, and Pacchionian granulations or bodies) are small outpouchings of the arachnoid mater and subarachnoid space into the dural venous sinuses of the brain. The granulations are thought to mediate the draining of cerebrospinal fluid (CSF) from the subarachnoid space into the venous system.

The largest and most numerous granulations lie along the superior sagittal sinus; they are however present along other dural sinuses as well.

Anatomy

The granulations are often situated near where cerebral veins drain into the dural sinuses. They are most prominent along the superior sagittal sinus, particularly those lodged within the lateral lacunae. In order of decreasing frequency, the granulations occur within the: superior sagittal sinus, transverse sinuses, superior petrosal sinuses, and straight sinus.

The arachnoid granulations may be lodged within granular foveae — small pits upon the inner surface of the cranial bones.

Structure

The arachnoid granulations are local outpouchings of the arachnoid mater, as well as the subarachnoid space enclosed within it, into the dural venous sinuses. The granulations exhibit a thinner stalk that penetrates through the wall of a venous sinus, and a distended head formed within the lumen of the sinus. The head consists of a trabecular collagenous core that is largely covered by a dural cupula, except for an apical cap — some 0.3 mm in diameter — of arachnoid cells attached directly to the dural venous endothelium.