Editing Arachnoid granulation

{{short description|Protrusions of the arachnoid mater for returning cerebrospinal fluid to circulation}}
{{Infobox anatomy
| Name        = Arachnoid granulation
| Latin       = granulationes arachnoideae
| Image       = Gray769-en.svg
| Caption     = 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.)
| Image2      = Arachnoid granulations.jpg
| Caption2    = Arachnoid granulations seen on [[autopsy]], where the dura mater has been removed but the arachnoid mater is left in place.
| System      =
}}
'''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|cerebrospinal fluid (CSF)]] from the [[subarachnoid space]] into the [[venous system]].<ref>{{cite book |last1=Hall |first2=John E. |last2=Hall |first1=Michael E. |title=Guyton and Hall textbook of medical physiology |year=2021 |publisher=Elsevier |location=Philadelphia, PA |isbn=978-0-323-59712-8 |edition=14th |pages=781–782}}</ref>

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 lacuna|lateral lacunae]]. In order of decreasing frequency, the granulations occur within the: superior sagittal sinus, [[transverse sinuses]], [[Superior petrosal sinus|superior petrosal sinuses]], and [[straight sinus]].<ref name="Standring-2020">{{Cite book |last=Standring |first=Susan |url=https://www.worldcat.org/oclc/1201341621 |title=Gray's Anatomy: The Anatomical Basis of Clinical Practice |publisher=[[Elsevier]] |year=2020 |isbn=978-0-7020-7707-4 |edition=42th |location=New York |pages=413 |oclc=1201341621}}</ref>

The arachnoid granulations may be lodged within '''granular foveae''' — small pits upon the inner surface of the cranial bones.<ref name="Edwards1934">{{cite book |author=Linden Forest Edwards |url=https://books.google.com/books?id=AfzaAAAAMAAJ |title=Anatomy for physical education, descriptive and applied |publisher=P. Blakiston's son & co., inc. |year=1934 |page=80 |access-date=23 June 2012}}</ref><ref name="Morris1921">{{cite book |author=Sir Henry Morris |url=https://archive.org/details/morrisshumanana00jackgoog |title=Morris's human anatomy |publisher=P. Blakiston's son & Company |year=1921 |page=[https://archive.org/details/morrisshumanana00jackgoog/page/n976 953] |access-date=23 June 2012}}</ref>

=== 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]].<ref name="Standring-2020" />

=== Development ===
The granulations develop during childhood as separate [[arachnoid villi]] gradually aggregate into macroscopic clumps.'''<ref>{{Cite book |last=Sinnatamby |first=Chummy S. |title=Last's Anatomy |publisher= Elsevier Australia|year=2011 |isbn=978-0-7295-3752-0 |edition=12th |pages=440}}</ref>'''

==Function==
The arachnoid granulations are notably thought to be involved in resorption of [[cerebrospinal fluid]], however, their function is not entirely understood.<ref name="Standring-2020" />

=== Cerebrospinal fluid resorption ===
The arachnoid granulations act as one-way [[valve]]s. Normally the [[pressure]] of the CSF is higher than that of the [[venous system]], so CSF flows through the granulations into the blood. If the pressure is reversed for some reason, fluid will not pass back into the subarachnoid space.<ref name="Spierer2023">{{cite journal |last1=Spierer |first1=R |title=The debated neuroanatomy of the fourth ventricle. |journal=Journal of Anatomy |date=October 2023 |volume=243 |issue=4 |pages=555–563 |doi=10.1111/joa.13885 |pmid=37170923|pmc=10485575 }}</ref> The reason for this is not known. It has been suggested that the [[endothelial cell]]s of the venous sinus create [[vacuole]]s of CSF, which move through the cell and out into the blood.<ref>{{Cite journal |last1=McKnight |first1=Colin D. |last2=Rouleau |first2=Renee M. |last3=Donahue |first3=Manus J. |last4=Claassen |first4=Daniel O. |date=2020-10-19 |title=The Regulation of Cerebral Spinal Fluid Flow and Its Relevance to the Glymphatic System |url=https://doi.org/10.1007/s11910-020-01077-9 |journal=Current Neurology and Neuroscience Reports |language=en |volume=20 |issue=12 |pages=58 |doi=10.1007/s11910-020-01077-9 |issn=1534-6293 |pmc=7864223 |pmid=33074399}}</ref>

The importance of arachnoid granulations for the drainage of CSF is controversial.<ref name="Norwood-2019" /> The granulations are sparse during early life, possibly underscoring the importance of alternate mechanisms of drainage.<ref name="Standring-2020" /> A large portion (perhaps the majority) of CSF may in fact drain through lymphatics associated with extracranial segments of cranial nerves - especially through axons of CN I (olfactory nerve) through their extension through the [[cribriform plate]].<ref name="Norwood-2019">{{cite journal |last1=Norwood |first1=Jordan N |last2=Zhang |first2=Qingguang |last3=Card |first3=David |last4=Craine |first4=Amanda |last5=Ryan |first5=Timothy M |last6=Drew |first6=Patrick J |title=Anatomical basis and physiological role of cerebrospinal fluid transport through the murine cribriform plate |journal=eLife |date=7 May 2019 |volume=8 |pages=e44278 |doi=10.7554/eLife.44278 |pmid=31063132 |pmc=6524970 |doi-access=free }}</ref>

=== Subarachnoid systolic overpressure dampening ===
A suggested alternative or additional function of the granulation may be the dispersal of the overpressure wave formed within the subarachnoid space by the pulsation of arteries during systole. As the venous sinuses are enclosed in rigid dural structures, they represent a non-distensible compartment into which subarachnoid pressure increases may be dispersed.<ref name="Standring-2020" />

== Clinical significance ==
Age-related degenerative changes of the granulations and consequent decreased CSF resorption may underlie [[normal pressure hydrocephalus]] (which may in turn be pathogenetically implicated in additional age-related neurodegenerative disorders.<ref name="Standring-2020" />

==Eponym==
Occasionally, they are referred to by their old name: '''Pacchioni's bodies''', named after Italian anatomist [[Antonio Pacchioni]].<ref>{{WhoNamedIt|synd|392}}</ref>

==References==
<references/>

==Additional images==
{{Commons}}
<gallery>
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 sinuses|transverse sinus]]
File:Pacchioni-Granulation CT KF.jpg|Non-contrast CT scan of the head showing an arachnoid granulation
</gallery>

{{Meninges}}
{{Authority control}}

[[Category:Meninges]]