The arachnoid mater (or simply arachnoid) is one of the three
meninges
In anatomy, the meninges (; meninx ; ) 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 spac ...
, the protective
membrane
A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Membranes can be generally classified into synthetic membranes and biological membranes. Bi ...
s that cover the
brain
The brain is an organ (biology), organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It consists of nervous tissue and is typically located in the head (cephalization), usually near organs for ...
and
spinal cord
The spinal cord is a long, thin, tubular structure made up of nervous tissue that extends from the medulla oblongata in the lower brainstem to the lumbar region of the vertebral column (backbone) of vertebrate animals. The center of the spinal c ...
. It is so named because of its resemblance to a
spider web
A spider web, spiderweb, spider's web, or cobweb (from the archaic word ''Wikt:coppe, coppe'', meaning 'spider') is a structure created by a spider out of proteinaceous spider silk extruded from its spinnerets, generally meant to catch its prey ...
. The arachnoid mater is a derivative of the
neural crest
The neural crest is a ridge-like structure that is formed transiently between the epidermal ectoderm and neural plate during vertebrate development. Neural crest cells originate from this structure through the epithelial-mesenchymal transition, ...
mesoectoderm in the embryo.
Structure
The arachnoid mater is interposed between the two other meninges, the more superficial (closer to the surface) and much thicker dura mater and the deeper
pia mater
Pia mater ( or ),Entry "pia mater" in subarachnoid space. The delicate arachnoid layer is not attached to the inside of the dura but against it, and surrounds the brain and spinal cord. It does not line the brain down into its
sulci
Sulci or Sulki (in Greek , Stephanus of Byzantium, Steph. B., Ptolemy, Ptol.; , Strabo; , Pausanias (geographer), Paus.), was one of the most considerable cities of ancient Sardinia, situated in the southwest corner of the island, on a small isla ...
(folds), as does the pia mater, with the exception of the
longitudinal fissure
The longitudinal fissure (or cerebral fissure, great longitudinal fissure, median longitudinal fissure, interhemispheric fissure) is the deep groove that separates the two cerebral hemispheres of the vertebrate brain. Lying within it is a continu ...
, which divides the left and right
cerebral hemisphere
The vertebrate cerebrum (brain) is formed by two cerebral hemispheres that are separated by a groove, the longitudinal fissure. The brain can thus be described as being divided into left and right cerebral hemispheres. Each of these hemispheres ...
s.
Cerebrospinal fluid
Cerebrospinal fluid (CSF) is a clear, colorless Extracellular fluid#Transcellular fluid, transcellular body fluid found within the meninges, meningeal tissue that surrounds the vertebrate brain and spinal cord, and in the ventricular system, ven ...
(CSF) flows under the arachnoid in the subarachnoid space, within a meshwork of trabeculae which span between the arachnoid and the pia. The arachnoid mater makes arachnoid villi, small protrusions through the dura mater into the venous sinuses of the brain, which allow CSF to exit the subarachnoid space and enter the blood stream.
Unlike the dura mater, which receives a rich vascular supply from numerous arteries, the arachnoid mater is avascular (lacking blood vessels).
The arachnoid mater and dura mater are very close together throughout the cranium and spinal canal all the way to sacral vertebra S2, where the two layers fuse into one and end in the filum terminale, which attaches to the coccygeal end of the spinal canal. Sandwiched between the dura and arachnoid maters lie some veins that connect the brain's venous system with the venous system in the dura mater.
The arachnoid mater covering the brain is referred to as the ''arachnoidea encephali'', and the portion covering the spinal cord as the ''arachnoidea spinalis''. The arachnoid and pia mater are sometimes considered as a single structure, the leptomeninx, or the plural version, leptomeninges (''lepto'', from the Greek root meaning "thin" or "slender"). Similarly, the dura in this situation is called the pachymeninx.
There are two subdivisions of arachnoid mater surrounding the subarachnoid space, the dorsal layer and the ventral layer.
The dorsal layer covers internal cerebral veins and fixes them to the surrounding
tela choroidea
The tela choroidea (or tela chorioidea) is a region of meninges, meningeal pia mater that adheres to the underlying ependyma, and gives rise to the choroid plexus in each of the brain’s Ventricular system, four ventricles. ''Tela'' is Latin for ...
. The ventral layer of arachnoid membrane, on the other hand, is a direct anterior extension of this arachnoid envelope that the dorsal layer forms over the pineal region.
The arachnoid mater in the rat is composed of approximately 10 layers of
fibroblast
A fibroblast is a type of cell (biology), biological cell typically with a spindle shape that synthesizes the extracellular matrix and collagen, produces the structural framework (Stroma (tissue), stroma) for animal Tissue (biology), tissues, and ...
cells.
Function
CSF circulates in the subarachnoid space (between arachnoid and pia mater). Cerebrospinal fluid is produced by the
choroid plexus
The choroid plexus, or plica choroidea, is a plexus of cells that arises from the tela choroidea in each of the ventricles of the brain. Regions of the choroid plexus produce and secrete most of the cerebrospinal fluid (CSF) of the central ...
(inside the ventricles of the brain, which are in direct communication with the subarachnoid space so the CSF can flow freely through the nervous system).
Cerebrospinal fluid is a transparent, colourless fluid and it is produced at about 500 ml/day. Its electrolyte levels, glucose levels, and pH are very similar to those in plasma, but the presence of blood in cerebrospinal fluid is always abnormal.
Etymology
The arachnoid mater is named after the
Greek
Greek may refer to:
Anything of, from, or related to Greece, a country in Southern Europe:
*Greeks, an ethnic group
*Greek language, a branch of the Indo-European language family
**Proto-Greek language, the assumed last common ancestor of all kno ...
word '' arachne'' ("spider"), the suffix ''-oid'' ("in the image of"), and the Latin word ''mater'' ("mother"), because of the fine spider-web–like appearance of the delicate fibres of the arachnoid (
arachnoid trabeculae
The arachnoid trabeculae (AT) are delicate strands of connective tissue that loosely connect the two innermost layers of the meninges – the arachnoid mater and the pia mater.Frederik Ruysch in 1699. Another source states that the "arachnoid membrane" was discovered and named by Gerardus Blaes ( Blasius) in 1664, and that Ruysch adopted the term in 1692.
Additional images
File:Meninges-en.svg, Meninges
File:Gray1196.png, Diagrammatic section of scalp.
file:Sobo 1909 590.png, The arachnoid mater lies under the dura mater, and arteries and veins run on top of it.
File:Brain with arachnoid and pia mater.jpg, Brain with arachnoid mater, and an area where it is removed, showing cerebral gyri covered by the translucent pia mater.
File:Spinal_dura_mater_1.jpg, Spinal dura mater opened, arachnoid mater visible.
File:Gray767.png, The medulla spinalis and its membranes.
File:Slide5sese.GIF, Spinal cord. Spinal membranes and nerve roots. Deep dissection. Posterior view.
File:Slide6Neo.JPG, Meninges and superficial cerebral veins. Deep dissection. Superior view.
File:Slide7Neo.JPG, Meninges and superficial cerebral veins. Deep dissection. Superior view.
The arachnoid cells continue inside the brain, covering the so-called Virchow-Robin spaces or
perivascular space
A perivascular space, also known as a Virchow–Robin space, is a fluid-filled space surrounding certain blood vessels in several organs, including the brain, potentially having an immune system, immunological function, but more broadly a dis ...
s. For that reason some meningiomas can appear as completely inside the brain.