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Meninges
Three membranes protecting the brain From Wikipedia, the free encyclopedia
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In anatomy, the meninges (/məˈnɪndʒiːz/;[1][2] sg. meninx /ˈmiːnɪŋks, ˈmɛnɪŋks/;[3] from Ancient Greek μῆνιγξ (mêninx) 'membrane')[4] are protective membranes that cover the brain and spinal cord. In mammals, three meninges have been clearly identified: the dura mater, the arachnoid mater, and the pia mater. Each layer has its own molecularly distinct type of fibroblasts.[5] The primary function of the meninges is to protect the central nervous system.[6]
The dura mater envelops the arachnoid mater and supports the dural sinuses carrying blood from the brain toward the heart.[7] The area between the arachnoid mater and the pia mater is known as the subarachnoid space. It contains cerebrospinal fluid. The arachnoid and pia maters produce prostaglandin D2 synthase, a major cerebrospinal fluid protein.[8] The arachnoid mater provides a restrictive permeability barrier between the cerebrospinal fluid in the subarachnoid space and the blood circulation in the dura.[5] The pia mater is a thin sheet of connective tissue that interfaces with the glial limitans superficialis.[9]
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Structures
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Dura mater
The dura mater (Latin: tough mother),[10][a] is a thick, durable membrane, closest to the skull and vertebrae. The dura mater, the outermost part, is dense fibrous tissue formed from an interlay of collagen fibers, elastin, and fibroblasts in an unformed extracellular matrix[11]. The dura mater is itself a two layered membrane: an outer endosteal (periosteal) layer lies closest to the skull, and an inner (meningeal or dura mater proper) layer lies closer to the brain.[12][9] These layers separate to surround the dural venous sinuses. Sensory and autonomic nerves innervate the dura, and are dense near its blood vessels.[9] The dura's inner surface is covered by flattened fibrocytes which are adhered to by the outer cells of the arachnoid mater.[13] The dura mater envelops the arachnoid mater and surrounds and supports the large dural sinuses carrying blood from the brain toward the heart.[7]
The dura has four areas of infolding:[14][15]
- Falx cerebri, the largest, sickle-shaped; separates the cerebral hemispheres. Starts from the frontal crest of frontal bone and the crista galli running to the internal occipital protuberance.[14]
- Tentorium cerebelli, the second largest, crescent-shaped; separates the occipital lobes from cerebellum. The falx cerebri attaches to it giving a tentlike appearance.[14]
- Falx cerebelli, vertical infolding; lies inferior to the tentorium cerebelli, separating the cerebellar hemispheres.[14]
- Diaphragma sellae, smallest infolding; covers the pituitary gland and sella turcica.[14]
Arachnoid mater
The middle element of the meninges is the arachnoid mater, or arachnoid membrane, so named because of its resemblance to a spider web. It cushions the central nervous system. This thin, transparent membrane is composed of fibrous tissue and, like the pia mater, has an outer layer of tightly packed flat cells, forming the arachnoid barrier.[16]
The shape of the arachnoid does not follow the convolutions of the surface of the brain and so looks like a loosely fitting sac. In particular, in the region of the brain a large number of fine filaments called arachnoid trabeculae pass from the arachnoid through the subarachnoid space to blend with the tissue of the pia mater.[17] The arachnoid barrier creates a restrictive permeability barrier between the cerebrospinal fluid in the subarachnoid space and the blood circulation in the dura.[5]
The arachnoid barrier layer is characterized by a distinct continuous basal lamina on its inner surface toward the innermost collagenous portion of the arachnoid reticular layer.[18]
Pia mater
The pia mater (Latin: tender mother)[19] is a very delicate membrane. It is the meningeal envelope that firmly adheres to the surfaces of the brain and spinal cord,[9] following all of the brain's contours (gyri and sulci).[20] It is a very thin sheet of connective tissue that interfaces with the glial limitans superficialis but lacks capillaries itself.[9]
Subarachnoidal lymphatic-like membrane
The subarachnoid lymphatic-like membrane (SLYM) is a possible fourth meningeal layer that was proposed in 2023 in the brain of humans and mice.[21] Its existence is still very controversial.
The SLYM would be located in the subarachnoid space, the space between the middle reticular meninges and the innermost tender meninges that lie close to the brain.[21] It divides the subarachnoid space into an outer, superficial compartment and an inner, deeper area surrounding the brain.[21]Leptomeninges
The arachnoid and pia mater are sometimes together called the leptomeninges,[22] literally "thin meninges" (Greek: λεπτός "leptos"—"thin"). Acute meningococcal meningitis can lead to an exudate within the leptomeninges along the surface of the brain.[23] Because the arachnoid is connected to the pia by cobweb-like strands, it is structurally continuous with the pia, hence the name pia-arachnoid or leptomeninges. They are responsible for the production of beta-trace protein (prostaglandin D2 synthase), a major cerebrospinal fluid protein.[8]
Subarachnoid space
The subarachnoid space is the space that normally exists between the arachnoid and the pia mater. It is filled with cerebrospinal fluid and continues down the spinal cord. Spaces are formed from openings at different points along the subarachnoid space; these are the subarachnoid cisterns, which are filled with cerebrospinal fluid.[24]
The dura mater is attached to the skull,[12] whereas in the spinal cord, the dura mater is separated from the vertebrae by a space called the epidural space, which contains fat and blood vessels. The arachnoid is attached to the dura mater, while the pia mater is attached to the central nervous system tissue. If the dura mater and the arachnoid become separated due to injury or illness, the space between them is known as the subdural space.[25][26] There is another potential space, the subpial space, between the pia mater and the glia limitans.[27]
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Clinical significance
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Three types of intracranial hemorrhage can involve the meninges: epidural, subdural, and subarachnoid.[28]
- A subarachnoid hemorrhage is acute bleeding under the arachnoid; it may occur spontaneously or as a result of trauma.[29][28]
- A subdural hematoma (SDH) is an extracerebral collection of blood located in the potential space that can separate arachnoid from the dura mater. The origin is usually venous, caused by injury to the bridging veins that connect the dura mater and the arachnoid. Once these are torn, blood leaks into this area. SDHs occur in about 30% cases of severe head trauma.[28]
- An epidural hematoma (EDH) is an extracerebral collection of blood between the skull and the dura mater, underlying a bare bone surface. It is often associated with skull fracture. EDH may be arterial (caused by injury of a meningeal artery) or venous (related to damage to of a dural venous sinus or bleeding from diploic veins).[28]
Other medical conditions that affect the meninges include meningitis, which usually arises from a viral, bacterial, or fungal infection.[30][31]
Meningiomas are the most common type of primary brain tumors to occur in adults. They are thought to arise from meningothelial arachnoid cells in the meninges. Most commonly they attach firmly to the inner surface of the dura and are well-circumscribed; but some tumors may show brain invasion.[32] More rarely, leptomeningeal cancers may metastasize from tumors elsewhere in the body to the cerebrospinal fluid and leptomeninges.[33]
Migraine has been linked to the meninges and the activity of calcitonin gene-related peptide (CGRP) in the trigeminal ganglion and central nervous system, according to preliminary research.[9] Stimulation of the trigeminal nerve may result in release of neuropeptides such as CGRP, vasodilation of cerebral and dural blood vessels, neurogenic inflammation, and the transmission of pain signals via nerves in the meninges.[9]
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Other animals
In fish, there is a single membrane known as the primitive meninx.[34] Amphibians and reptiles have two meninges, and birds and mammals have three.[34] Mammals (as higher vertebrates) retain the dura mater, and the secondary meninx divides into the arachnoid and pia mater.[35]
History
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The first known reference to the dura appears in Egypt, in Case 6 of the Edwin Smith Papyrus. Hippocrates described the dura in his monograph “On Injuries of the Head” and insisted that care should be taken to keep it intact and clean.[36] Celsus agreed, and described a method of treatment for depressed fractures.[37] Galen was the first to describe the pia mater in humans in the second century AD.[38]
The arachnoid layer was first described by Dutch physician Gerardus Blasius in 1664.[38] In 1695, Humphrey Ridley first described the subarachnoid cisterns. He also contributed to the understanding of the blood-brain barrier, and accurately described the fifth cranial nerve ganglion with its branches.[39] In 1699, Frederick Ruysch confirmed that the arachnoid mater formed a complete layer that surrounded the brain. Its current name is based on his description of its spiderlike morphology.[40] Arachnoid granulations were first described by Italian physician Antonio Pacchioni who published his Dissertatio Epistolaris de Glandulis Conglobatis Durae Meningis Humanae in 1705.[41]
In seven articles from 1899 to 1902, Italian anatomist Giuseppe Sterzi described comparative studies on the meninges from the lancelet to the human. He showed that the spinal meninges were very simple in adult lower vertebrates and in the early development of more advanced vertebrates.[42]
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See also
Notes
- Also rarely called meninx fibrosa or pachymeninx
References
External links
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