Arachnoid granulation | |
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![]() 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.) | |
![]() Arachnoid granulations seen on autopsy, where the dura mater has been removed but the arachnoid mater is left in place. | |
Details | |
Identifiers | |
Latin | granulationes arachnoideae |
TA98 | A14.1.01.205 |
TA2 | 5389 |
FMA | 77760 |
Anatomical terminology |
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 (CSF) from the subarachnoid space into the venous system.
The largest and most numerous granulations lie along the superior sagittal sinus; they are however present along other dural sinuses as well.
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 withinin the lateral lacunae. In order of decreasing frequency, the granulations occur within the: superior sagittal sinus, transverse sinuses, superior petrosal sinuses, and straight sinus. [1]
The arachnoid granulations may be lodged within granular foveae - small pits upon the inner surface of the cranial bones. [2] [3]
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 directl to the dural venous endothelium. [1]
The granulations develop during childhood as separate arachnoid villi gradually aggregate into macroscopic clumps. [4]
The arachnoid granulations are notably thought to be involved in resorption of cerebrospinal fluid, however, their function is not entirely understood. [1]
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 not pass back into the subarachnoid space. 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. [5]
The importance of arachnoid granulations for the drainage of CSF is controversial. [6] The granulations are sparse during early life, possibly underscoring the importance of alternate mechanisms of drainage. [1] 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. [6]
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. [1]
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. [1]
Occasionally, they are referred to by their old name: Pacchioni's granulations or pacchionian bodies, named after Italian anatomist Antonio Pacchioni. [7]
Cerebrospinal fluid (CSF) is a clear, colorless body fluid found within the tissue that surrounds the brain and spinal cord of all vertebrates.
Hydrocephalus is a condition in which an accumulation of cerebrospinal fluid (CSF) occurs within the brain. This typically causes increased pressure inside the skull. Older people may have headaches, double vision, poor balance, urinary incontinence, personality changes, or mental impairment. In babies, it may be seen as a rapid increase in head size. Other symptoms may include vomiting, sleepiness, seizures, and downward pointing of the eyes.
In neuroanatomy, the ventricular system is a set of four interconnected cavities known as cerebral ventricles in the brain. Within each ventricle is a region of choroid plexus which produces the circulating cerebrospinal fluid (CSF). The ventricular system is continuous with the central canal of the spinal cord from the fourth ventricle, allowing for the flow of CSF to circulate.
In anatomy, the meninges are the three membranes that envelop the brain and spinal cord. In mammals, the meninges are the dura mater, the arachnoid mater, and the pia mater. Cerebrospinal fluid is located in the subarachnoid space between the arachnoid mater and the pia mater. The primary function of the meninges is to protect the central nervous system.
Pia mater, often referred to as simply the pia, is the delicate innermost layer of the meninges, the membranes surrounding the brain and spinal cord. Pia mater is medieval Latin meaning "tender mother". The other two meningeal membranes are the dura mater and the arachnoid mater. Both the pia and arachnoid mater are derivatives of the neural crest while the dura is derived from embryonic mesoderm. The pia mater is a thin fibrous tissue that is permeable to water and small solutes. The pia mater allows blood vessels to pass through and nourish the brain. The perivascular space between blood vessels and pia mater is proposed to be part of a pseudolymphatic system for the brain. When the pia mater becomes irritated and inflamed the result is meningitis.
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 system. The other two meningeal layers are the arachnoid mater and the pia mater. It envelops the arachnoid mater, which is responsible for keeping in the cerebrospinal fluid. It is derived primarily from the neural crest cell population, with postnatal contributions of the paraxial mesoderm.
The great cerebral vein is one of the large blood vessels in the skull draining the cerebrum of the brain. It is also known as the vein of Galen, named for its discoverer, the Greek physician Galen.
Intracranial hemorrhage (ICH), also known as intracranial bleed, is bleeding within the skull. Subtypes are intracerebral bleeds, subarachnoid bleeds, epidural bleeds, and subdural bleeds.
In anatomy, the epidural space is the potential space between the dura mater and vertebrae (spine).
Antonio Pacchioni was an Italian scientist and anatomist, who focused chiefly on the outermost meningeal layer of the brain, the dura mater.
The median aperture is an opening of the fourth ventricle at the caudal portion of the roof of the fourth ventricle. It allows flow of cerebrospinal fluid (CSF) from the fourth ventricle into the cisterna magna. The other two openings of the fourth ventricle are the lateral apertures - one on either side. Nonetheless, the median aperture accounts for most of the outflow of CSF out of the fourth ventricle. The median aperture varies in size.
The falx cerebri is a large, crescent-shaped fold of dura mater that descends vertically into the longitudinal fissure between the cerebral hemispheres of the human brain, separating the two hemispheres and supporting dural sinuses that provide venous and CSF drainage to the brain. It is attached to the crista galli anteriorly, and blends with the tentorium cerebelli posteriorly.
The arachnoid mater is one of the three meninges, the protective membranes that cover the brain and spinal cord. It is so named because of its resemblance to a spider web. The arachnoid mater is a derivative of the neural crest mesoectoderm in the embryo.
The cavernous sinus within the human head is one of the dural venous sinuses creating a cavity called the lateral sellar compartment bordered by the temporal bone of the skull and the sphenoid bone, lateral to the sella turcica.
The dural venous sinuses are venous sinuses (channels) found between the endosteal and meningeal layers of dura mater in the brain. They receive blood from the cerebral veins, and cerebrospinal fluid (CSF) from the subarachnoid space via arachnoid granulations. They mainly empty into the internal jugular vein. Cranial venous sinuses communicate with veins outside the skull through emissary veins. These communications help to keep the pressure of blood in the sinuses constant.
The confluence of sinuses, torcular Herophili, or torcula is the connecting point of the superior sagittal sinus, straight sinus, and occipital sinus. It is below the internal occipital protuberance of the skull. It drains venous blood from the brain into the transverse sinuses. It may be affected by arteriovenous fistulas, a thrombus, major trauma, or surgical damage, and may be imaged with many radiology techniques.
The superior sagittal sinus, within the human head, is an unpaired area along the attached margin of the falx cerebri. It allows blood to drain from the lateral aspects of anterior cerebral hemispheres to the confluence of sinuses. Cerebrospinal fluid drains through arachnoid granulations into the superior sagittal sinus and is returned to venous circulation.
A cerebrospinal fluid leak is a medical condition where the cerebrospinal fluid (CSF) that surrounds the brain and spinal cord leaks out of one or more holes or tears in the dura mater. A CSF leak is classed as either spontaneous (primary), having no known cause, or nonspontaneous (secondary) where it is attributed to an underlying condition. Causes of a primary CSF leak are those of trauma including from an accident or intentional injury, or arising from a medical intervention known as iatrogenic. A basilar skull fracture as a cause can give the sign of CSF leakage from the ear nose or mouth. A lumbar puncture can give the symptom of a post-dural-puncture headache.
The glymphatic system is a system for waste clearance in the central nervous system (CNS) of vertebrates. According to this model, cerebrospinal fluid (CSF) flows into the paravascular space around cerebral arteries, combining with interstitial fluid (ISF) and parenchymal solutes, and exiting down venous paravascular spaces. The pathway consists of a para-arterial influx route for CSF to enter the brain parenchyma, coupled to a clearance mechanism for the removal of interstitial fluid (ISF) and extracellular solutes from the interstitial compartments of the brain and spinal cord. Exchange of solutes between CSF and ISF is driven primarily by arterial pulsation and regulated during sleep by the expansion and contraction of brain extracellular space. Clearance of soluble proteins, waste products, and excess extracellular fluid is accomplished through convective bulk flow of ISF, facilitated by astrocytic aquaporin 4 (AQP4) water channels.
The meningeal lymphatic vessels are a network of conventional lymphatic vessels located parallel to the dural venous sinuses and middle meningeal arteries of the mammalian central nervous system (CNS). As a part of the lymphatic system, the meningeal lymphatics are responsible for draining immune cells, small molecules, and excess fluid from the CNS into the deep cervical lymph nodes. Cerebrospinal fluid, and interstitial fluid are exchanged, and drained by the meningeal lymphatic vessels.