Arachnoid granulations

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Definition, location and structure of the arachnoid mater.

Arachnoid mater forms the middle layer of the cranial meninges and lies directly beneath the dura mater. Between the arachnoid and pia mater is a space known as the subarachnoid space which houses cerebrospinal fluid. 

Extending from the subarachnoid space to the dural venous sinuses of the brain are small outpouchings or protrusions known as arachnoid granulations. Arachnoid granulations are pockets of arachnoid membrane and subarachnoid space which connect cerebrospinal fluid to venous circulation. They extend from the subarachnoid space through the dura mater to enter the walls of dural venous sinuses and allow for the passage of cerebrospinal fluid.

A thin layer of arachnoid membrane extends through the dural layer and expands to form a bulbous core of collagenous trabeculae and a series of interwoven channels. The apex of each arachnoid granulation is enclosed with a layer of arachnoid cells and overlying endothelium. Interwoven cerebrospinal fluid channels extend through the arachnoid cap to reach the subendothelial regions of each granulation.

Cerebrospinal fluid from the subarachnoid space flows into the arachnoid granulations, through its channels and diffuses across the endothelium of the venous sinus to enter the venous system. Arachnoid granulations act as one way valves and therefore venous circulation does not enter the subarachnoid spaces. 

Arachnoid granulations are most commonly found within the transverse and superior sagittal sinuses of the brain and are typically present close to points where veins enter the dural venous sinuses.