What are arachnoid granulations?
Also known as the arachnoid villi, the arachnoid granulation refers to the small protrusions of the arachnoid that are found through the dura mater. The arachnoid is the second membranous layer that covers the brain, while the dura mater is the outer protective layer. As they enter into the venous sinuses of the brain, these small protrusions have a very important function. Basically, they allow for the cerebrospinal fluid to enter into the bloodstream, after leaving the subarachnoid space.The bigger arachnoid granulation can be found along the superior sagittal sinus. This is actually a large venous space, which can be found inside the skull, in the center area (going from the front to the back of the skull). Other dural sinuses present arachnoid granulations as well. It is important to remember that the size of the arachnoid granulation increases with age, becoming calcified at the same time.In general, the size of the protrusions dictates their name. While the smaller arachnoid granulations are known as villi, the ones that are bigger in size and also calcified are often presented as pacchionian bodies (this name was used to describe all of the arachnoid granulations in the past – it comes from the Italian anatomist who discovered them, meaning Antonio Pacchioni; he was also the one who worked extensively on the research involving the anatomy of the dura mater). The analysis of the inside surface of the cranial bones has revealed the granular fovea; these are actually small pits which are produced by the arachnoid granulations.Scientists have been trying to find a connection between the arachnoid granulations and the appearance of hydrocephalus (no success so far). Many suspicions are related to the appearance of hydrocephalus, as a result of arachnoid granulation malfunction. Despite the extensive research in the field, it is still not certain when the arachnoid granulations start to function in a baby. During the prenatal period, their function is somewhat reduced and it seems that these become more active after the actual birth.
Arachnoid granulations function as one-way valves. In general, the cerebrospinal fluid has a higher pressure than the one of the venous system. This means that the cerebrospinal fluid will actually use these protrusions in order to enter the bloodstream. In the situation that the pressure levels are reversed, the cerebrospinal fluid will return into the brain, meaning in the subarachnoid space. At the moment, it cannot be said with certainty why such a thing happens. One of the suggestions that have been made regards the implication of the venous sinus. It has been discovered that the endothelial cells of the venous sinus actually lead to the appearance of cerebrospinal fluid vacuoles. What happens is that these vacuoles move through the respective cells and further into the bloodstream.It cannot be said with certainty how big of a role the arachnoid granulations play in the drainage of the cerebrospinal fluid. In general, it is known that a large percentage of the cerebrospinal fluid is drained through the lymphatic system. This is often associated with the cranial nerves and, more particularly, with their extracranial segments. On the other hand, a big part of the cerebrospinal fluid is drained through the axons of the olfactory nerve. The cerebrospinal fluid leaves the cranial vault through the extension of these axons (cribriform plate).In conclusion, the arachnoid granulation serves to limit the flow of the cerebrospinal fluid into any of the dural sinuses. At the same time, it works to prevent the reflux of venous blood into the subarachnoid space.
The arachnoid granulations often have a parasagittal location, leading to specific changes on the X-rays of the skull, such as the osteolytic lucency or the filling defect that appears at the level of the dural venous sinuses. Because of the latter modification, it is possible to mistake the arachnoid granulation for dural venous thrombosis. The MRI can be used for the identification of these small protrusions, with general signal characteristics for the cerebrospinal fluid.
Pictures of Arachnoid granulations
Arachnoid granulation Picture 1 - Superior sagittal sinus, Subarachnoid granulation, Emissary vein, Skin, Skull, Perivascular space, Dura mater (falx cerebri), Subarachnoid space, Pia mater, Superior cerebral vein and Cerebral artery.
Arachnoid granulation Picture 2 - Diagram showing the section across the top of the skull showing different membranes of the brain.