In
anatomy, the meninges (, ''singular:'' meninx ( or ), ) are the three
membranes that envelop the
brain and
spinal cord. In
mammal
Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...
s, the meninges are the
dura mater, the
arachnoid mater
The arachnoid mater (or simply arachnoid) 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 c ...
, 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.
Structure
Dura mater
The
dura mater ( la, tough mother) (also rarely called ''meninx fibrosa'' or ''pachymeninx'') is a thick, durable membrane, closest to the
skull and vertebrae. The dura mater, the outermost part, is a loosely arranged, fibroelastic layer of cells, characterized by multiple interdigitating cell processes, no extracellular collagen, and significant extracellular spaces. The middle region is a mostly fibrous portion. It consists of two layers: the
endosteal
The endosteum (plural endostea) is a thin vascular membrane of connective tissue that lines the inner surface of the bony tissue that forms the medullary cavity of long bones.
This endosteal surface is usually resorbed during long periods of ma ...
layer, which lies closest to the
skull, and the inner meningeal layer, which lies closer to the brain.
It contains larger blood vessels that split into the capillaries in the
pia mater. It is composed of dense fibrous tissue, and its inner surface is covered by flattened cells like those present on the surfaces of the pia mater and arachnoid mater. The dura mater is a sac that envelops the arachnoid mater and surrounds and supports the large
dural sinuses
The dural venous sinuses (also called dural sinuses, cerebral sinuses, or cranial sinuses) are venous channels found between the endosteal and meningeal layers of dura mater in the brain. They receive blood from the cerebral veins, receive cerebr ...
carrying blood from the brain toward the heart.
The dura has four areas of infolding:
*
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.
*
Tentorium cerebelli, the second largest, crescent-shaped; separates the
occipital lobes from
cerebellum
The cerebellum (Latin for "little brain") is a major feature of the hindbrain of all vertebrates. Although usually smaller than the cerebrum, in some animals such as the mormyrid fishes it may be as large as or even larger. In humans, the cerebel ...
. The falx cerebri attaches to it giving a tentlike appearance.
*
Falx cerebelli, vertical infolding; lies inferior to the tentorium cerebelli, separating the
cerebellar hemisphere
The cerebellum consists of three parts, a median and two lateral, which are continuous with each other, and are substantially the same in structure. The median portion is constricted, and is called the vermis, from its annulated appearance which ...
s.
*
Diaphragma sellae, smallest infolding; covers the
pituitary gland and
sella turcica.
Arachnoid mater
The middle element of the meninges is the
arachnoid mater
The arachnoid mater (or simply arachnoid) 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 c ...
, 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.
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. The arachnoid barrier has no extracellular collagen and is considered to represent an effective morphological and physiological meningeal barrier between the cerebrospinal fluid in the subarachnoid space and the blood circulation in the dura.
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.
Pia mater
The
pia mater ( la, tender mother
) is a very delicate membrane. It is the meningeal envelope that firmly adheres to the surfaces of the brain and spinal cord, following all of the brain's contours (
gyri
In neuroanatomy, a gyrus (pl. gyri) is a ridge on the cerebral cortex. It is generally surrounded by one or more sulci (depressions or furrows; sg. ''sulcus''). Gyri and sulci create the folded appearance of the brain in humans and other ma ...
and
sulci). It is a very thin membrane composed of fibrous tissue covered on its outer surface by a sheet of flat cells thought to be impermeable to fluid. The pia mater is pierced by blood vessels to the brain and spinal cord, and its
capillaries nourish the brain.
Leptomeninges
The
arachnoid
Arachnoid may refer to:
* Relating to arachnids
* Arachnoid (astrogeology), a geological structure found only on the planet Venus
* Arachnoid (botany), referring to organs with a cobwebby exterior appearance
* Arachnoid granulation, small protr ...
and
pia mater are sometimes together called the ''leptomeninges'', literally "thin meninges" ( el, λεπτός "leptos"—"thin"). Acute
meningococcal meningitis
Meningococcal disease describes infections caused by the bacterium ''Neisseria meningitidis'' (also termed meningococcus). It has a high mortality rate if untreated but is vaccine-preventable. While best known as a cause of meningitis, it can a ...
can lead to an
exudate
An exudate is a fluid emitted by an organism through pores or a wound, a process known as exuding or exudation.
''Exudate'' is derived from ''exude'' 'to ooze' from Latin ''exsūdāre'' 'to (ooze out) sweat' (''ex-'' 'out' and ''sūdāre'' 'to ...
within the leptomeninges along the surface of the brain. Because the arachnoid is connected to the pia by cob-web 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
Prostaglandin-H2 D-isomerase (PTGDS) is an enzyme that in humans is encoded by the ''PTGDS'' gene.
Function
The protein encoded by this gene is a glutathione-independent prostaglandin D synthase that catalyzes the conversion of prostaglandin ...
), a major cerebrospinal fluid protein.
Subarachnoid space
The subarachnoid space is the space that normally exists between the
arachnoid
Arachnoid may refer to:
* Relating to arachnids
* Arachnoid (astrogeology), a geological structure found only on the planet Venus
* Arachnoid (botany), referring to organs with a cobwebby exterior appearance
* Arachnoid granulation, small protr ...
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
The subarachnoid cisterns are spaces formed by openings in the subarachnoid space, an anatomic space in the meninges of the brain. The space is situated between the two meninges, the arachnoid mater and the pia mater. These cisterns are filled wit ...
, which are filled with cerebrospinal fluid.
The
dura mater is attached to the
skull, 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. When the dura mater and the arachnoid separate through injury or illness, the space between them is the
subdural space. There is a subpial space underneath the pia mater that separates it from the
glia limitans
The glia limitans, or the glial limiting membrane, is a thin barrier of astrocyte foot processes associated with the parenchymal basal lamina surrounding the brain and spinal cord. It is the outermost layer of neural tissue, and among its respons ...
.
Clinical significance
Injuries involving the meninges, can result in a
hemorrhage and two types of
hematoma
A hematoma, also spelled haematoma, or blood suffusion is a localized bleeding outside of blood vessels, due to either disease or trauma including injury or surgery and may involve blood continuing to seep from broken capillary, capillaries. A he ...
.
* A
subarachnoid hemorrhage
Subarachnoid hemorrhage (SAH) is bleeding into the subarachnoid space—the area between the arachnoid membrane and the pia mater surrounding the brain. Symptoms may include a severe headache of rapid onset, vomiting, decreased level of consci ...
is acute bleeding under the arachnoid; it may occur spontaneously or as a result of trauma.
* A
subdural hematoma is a
hematoma
A hematoma, also spelled haematoma, or blood suffusion is a localized bleeding outside of blood vessels, due to either disease or trauma including injury or surgery and may involve blood continuing to seep from broken capillary, capillaries. A he ...
(collection of blood) located in a separation of the
arachnoid
Arachnoid may refer to:
* Relating to arachnids
* Arachnoid (astrogeology), a geological structure found only on the planet Venus
* Arachnoid (botany), referring to organs with a cobwebby exterior appearance
* Arachnoid granulation, small protr ...
from the
dura mater. The
bridging vein Bridging veins are veins in the subarachnoid space that puncture the dura mater and empty into the dural venous sinuses. A rupture of a bridging vein causes a subdural hematoma
A subdural hematoma (SDH) is a type of bleeding in which a Hematoma, ...
s that connect the
dura mater and the
arachnoid
Arachnoid may refer to:
* Relating to arachnids
* Arachnoid (astrogeology), a geological structure found only on the planet Venus
* Arachnoid (botany), referring to organs with a cobwebby exterior appearance
* Arachnoid granulation, small protr ...
are torn, usually during an accident, and blood leaks into this area.
* An
epidural hematoma, bleeding between the dura mater and the
skull, may arise after an accident or spontaneously.
Other medical conditions that affect the meninges include
meningitis
Meningitis is acute or chronic inflammation of the protective membranes covering the brain and spinal cord, collectively called the meninges. The most common symptoms are fever, headache, and neck stiffness. Other symptoms include confusion or ...
(usually from a
fungal,
bacterial, or
viral
Viral means "relating to viruses" (small infectious agents).
Viral may also refer to:
Viral behavior, or virality
Memetic behavior likened that of a virus, for example:
* Viral marketing, the use of existing social networks to spread a marke ...
infection) and
meningioma
Meningioma, also known as meningeal tumor, is typically a slow-growing tumor that forms from the meninges, the membranous layers surrounding the brain and spinal cord. Symptoms depend on the location and occur as a result of the tumor pressing o ...
s that arise from the meninges, or from
meningeal carcinomatoses (
tumors) that form elsewhere in the body and
metastasize to the meninges.
Other animals
In
fish, there is a single membrane known as the primitive meninx.
Amphibian
Amphibians are tetrapod, four-limbed and ectothermic vertebrates of the Class (biology), class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terres ...
s and
reptile
Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates (lizards and snakes) and rhynchocephalians ( ...
s have two meninges, and
birds and mammals have three.
In the early 1900s,
Giuseppe Sterzi
Giuseppe Nazzareno Sterzi (1876–1919) was an Italian anatomist, neuroanatomist and medical historian. Although his research activity encompassed no more than fifteen years, the themes treated by Sterzi are relevant to neuroanatomy and history ...
, an Italian anatomist, carried out comparative studies on the meninges from the
lancelet to the
human. Contrary to previous reports, the spinal meninges were seen to be very simple, both in the adult lower vertebrates and in the early developmental stages of the more advanced vertebrates. From the
mesenchyme
Mesenchyme () is a type of loosely organized animal embryonic connective tissue of undifferentiated cells that give rise to most tissues, such as skin, blood or bone. The interactions between mesenchyme and epithelium help to form nearly every o ...
surrounding the
neural tube, only a single leaflet forms the primitive meninx. In the following
phylogenetic and
ontogenetic stages, the latter divides into an internal leaflet: the secondary meninx, and into an external one: the dura mater. Finally, in higher vertebrates, even the secondary meninx divides into the arachnoid and the pia. In the same animals, Sterzi demonstrated that, while in the spinal medulla the dura keeps its identity, in the skull it fuses with the
periosteum. He also demonstrated the continuity of all meninges with the envelopes of nerves and with the
filum terminale.
Mammal
Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...
s (as higher vertebrates) retain the dura mater, and the secondary meninx divides into the
arachnoid
Arachnoid may refer to:
* Relating to arachnids
* Arachnoid (astrogeology), a geological structure found only on the planet Venus
* Arachnoid (botany), referring to organs with a cobwebby exterior appearance
* Arachnoid granulation, small protr ...
and
pia mater.
Additional images
File:Acta Eruditorum - IV fisiologia, 1703 – BEIC 13363829.jpg, Illustration of the work by Antonio Pacchioni ''Disquisitio anatomicae de durae meningis ...'' published in '' Acta Eruditorum'', 1703
See also
*
Cranial cavity
The cranial cavity, also known as intracranial space, is the space within the skull that accommodates the brain. The skull minus the mandible is called the ''cranium''. The cavity is formed by eight cranial bones known as the neurocranium that in ...
References
External links
*
{{Authority control
Back anatomy
Human head and neck