Arachnoid granulations (also arachnoid villi, and pacchionian granulations or bodies) are small protrusions of the arachnoid mater (the thin second layer covering the brain) into the outer membrane of the

dura mater In neuroanatomy, dura mater is a thick membrane made of dense irregular connective tissue that surrounds the brain and spinal cord. It is the outermost of the three layers of membrane called the meninges that protect the central nervous syste ...

(the thick outer layer). They protrude into the dural venous sinuses of the brain, and allow cerebrospinal fluid (CSF) 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 (on the inside of the

skull The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, t ...

). 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

valve A valve is a device or natural object that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or slurries) by opening, closing, or partially obstructing various passageways. Valves are technically fitting ...

s. Normally the

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country a ...

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 cell The endothelium is a single layer of squamous endothelial cells that line the interior surface of blood vessels and lymphatic vessels. The endothelium forms an interface between circulating blood or lymph in the lumen and the rest of the ves ...

s 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 (perhaps the majority) 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 (olfactory nerve) 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.

References

Additional images

File:Gray133.png, Left parietal bone. Inner surface. File:Gray135.png, Frontal bone. Inner surface. File:CT scan of a pacchionian body - transverse plane - labeled.jpg, CT angiography showing an arachnoid granulation in the right transverse sinus File:Pacchioni-Granulation CT KF.jpg, Non-contrast CT scan of the head showing an arachnoid granulation {{Authority control Meninges