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Arachnoid granulations (or arachnoid villi) are small protrusions of the arachnoid (the thin second layer covering the brain) through the dura mater (the thick outer layer). They protrude into the venous sinuses of the brain, and allow cerebrospinal fluid (CSF) to exit the sub-arachnoid space and enter the blood stream.

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). They are, however, present along other dural sinuses as well. Smaller granulations are called villi, large calcified ones are referred to as pacchionian bodies.

Function

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 pass back into the subarachnoid space (of the brain). 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 (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 cribiform plate.

On the inner surface of cranial bones, small pits called granular fovea are produced by arachnoid granulations.^[1]^[2]

Eponym

Occasionally, they are referred to by their old name: Pacchioni's granulations or pacchionian bodies, named after Italian anatomist Antonio Pacchioni.^[3]

References

^ Linden Forest Edwards (1934). Anatomy for physical education, descriptive and applied. P. Blakiston's son & co., inc. p. 80. Retrieved 23 June 2012.
^ Sir Henry Morris (1921). Morris's human anatomy. P. Blakiston's son & Company. p. 953. Retrieved 23 June 2012.
^ synd/392 at Who Named It?

Additional images

Left parietal bone. Inner surface.
Frontal bone. Inner surface.
CT scan of the head showing an arachnoid granulation

UpToDate Contents

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English Journal

Proposal of "evolution theory in cerebrospinal fluid dynamics" and minor pathway hydrocephalus in developing immature brain.

Oi S1, Di Rocco C.
Child's nervous system : ChNS : official journal of the International Society for Pediatric Neurosurgery.Childs Nerv Syst.2006 Jul;22(7):662-9. Epub 2006 May 10.
BACKGROUND: The specificity of cerebrospinal fluid (CSF) dynamics in the immature brain still remains unknown. In our data previously published, the transependymal intraparenchymal CSF pathway (the minor pathway) plays a significant role in various degrees in the alternative CSF passage. Now, there
PMID 16685545

[The relief of the luminal surface of the sinuses in the mammalian cranial dura mater].

Krasnikov IuA, Mironov AA.
Arkhiv anatomii, gistologii i émbriologii.Arkh Anat Gistol Embriol.1991 Sep-Oct;101(9-10):33-8.
In Mammalia with different types in organization of blood outflow in the dura mater venous sinuses: vertebral (tiger), jugular (fur-seal, cat, rabbit) and mixed (rat, dog, man) the internal surface relief of these sinuses has been studied. The total plan of the relief in all the species studied is p
PMID 1845512

Alternative pathways for drainage of cerebrospinal fluid in the canine brain.

Leeds SE1, Kong AK, Wise BL.
Lymphology.Lymphology.1989 Sep;22(3):144-6.
Although the brain has no formal lymphatic system, a substantial quantity of cerebrospinal fluid (CSF) has nonetheless been shown to drain via cervical lymphatics. To pursue further the issue of alternative drainage pathways for CSF, we infused a solution of Ringer's lactate (RL) into the cisterna m
PMID 2601407

「クモ膜顆粒」

Arachnoid granulation
	Diagrammatic representation of a section across the top of the skull, showing the membranes of the brain, etc. ("Arachnoid granulation" label is at top right.)
Details
Latin	Granulationes arachnoideae
Identifiers
Gray's	p.878
TA	A14.1.01.205
FMA	77760
	Anatomical terminology

　　[★]

英: arachnoid glanulation (M), arachnoid villus
ラ: granulationes arachnoidales
同: 脳膜顆粒、パッキオニ顆粒 Pacchioni顆粒パキオーニ顆粒 Pacchionian bodies、クモ膜絨毛 villi arachnoidales
関: クモ膜

[Edwards1934-1] Linden Forest Edwards (1934). Anatomy for physical education, descriptive and applied. P. Blakiston's son & co., inc. p. 80. Retrieved 23 June 2012.

[Morris1921-2] Sir Henry Morris (1921). Morris's human anatomy. P. Blakiston's son & Company. p. 953. Retrieved 23 June 2012.

[3] synd/392 at Who Named It?

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案内

案内

Pacchionian bodies