Pia mater ( or ),Entry "pia mater"
in
Merriam-Webster Online Dictionary
', retrieved 2012-07-28. often referred to as simply the pia, is the delicate innermost layer of the

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 membranes surrounding 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 ...

. ''Pia mater'' is medieval Latin meaning "tender mother". The other two meningeal membranes are the dura mater and 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 cr ...

. Both the pia and arachnoid mater are derivatives 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, ...

while the dura is derived from embryonic

mesoderm The mesoderm is the middle layer of the three germ layers that develops during gastrulation in the very early development of the embryo of most animals. The outer layer is the ectoderm, and the inner layer is the endoderm.Langman's Medical ...

. The pia mater is a thin

fibrous tissue Connective tissue is one of the four primary types of animal tissue, a group of cells that are similar in structure, along with epithelial tissue, muscle tissue, and nervous tissue. It develops mostly from the mesenchyme, derived from the mesode ...

that is permeable to water and small solutes. The pia mater allows blood vessels to pass through and nourish the brain. The

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 ...

between blood vessels and pia mater is proposed to be part of a pseudo

lymphatic system The lymphatic system, or lymphoid system, is an organ system in vertebrates that is part of the immune system and complementary to the circulatory system. It consists of a large network of lymphatic vessels, lymph nodes, lymphoid organs, lympha ...

for the brain (

glymphatic system The glymphatic system, glymphatic clearance pathway or paravascular system is an organ system for metabolic waste removal in the central nervous system (CNS) of vertebrates. According to this model, cerebrospinal fluid (CSF), an ultrafiltrated ...

). When the pia mater becomes irritated and inflamed the result is

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, intense headache, vomiting and neck stiffness and occasion ...

Structure

Pia mater is the thin, translucent, mesh-like meningeal envelope, spanning nearly the entire surface of the brain. It is absent only at the natural openings between the ventricles, the

median aperture The median aperture (median aperture of fourth ventricle or foramen of Magendie) is an opening at the caudal portion of the roof of the fourth ventricle. It allows the flow of cerebrospinal fluid (CSF) from the fourth ventricle into the cisterna ...

, and the

lateral aperture The lateral aperture, lateral aperture of fourth ventricle or foramen of Luschka (after anatomist Hubert von Luschka)Hube ...

. The pia firmly adheres to the surface of the brain and loosely connects to the arachnoid layer. Because of this continuum, the layers are often referred to as the pia arachnoid or leptomeninges. A subarachnoid space exists between the arachnoid layer and the pia, into which 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 ...

releases and maintains the cerebrospinal fluid (CSF). The subarachnoid space contains

trabeculae A trabecula (: trabeculae, from Latin for 'small beam') is a small, often microscopic, tissue element in the form of a small beam, strut or rod that supports or anchors a framework of parts within a body or organ. A trabecula generally has a ...

, or fibrous filaments, that connect and bring stability to the two layers, allowing for the appropriate protection from and movement of the proteins, electrolytes, ions, and glucose contained within the CSF. The thin membrane is composed of fibrous

connective tissue Connective tissue is one of the four primary types of animal tissue, a group of cells that are similar in structure, along with epithelial tissue, muscle tissue, and nervous tissue. It develops mostly from the mesenchyme, derived from the mesod ...

, which is covered by a sheet of flat cells impermeable to fluid on its outer surface. A network of blood vessels travels to the brain and spinal cord by interlacing through the pia membrane. These capillaries are responsible for nourishing the brain. This vascular membrane is held together by

areolar tissue Loose connective tissue, also known as areolar tissue, is a cellular connective tissue with thin and relatively sparse collagen fibers. They have a semi-fluid matrix with lesser proportions of fibers. Its ground substance occupies more volume ...

covered by

mesothelial The mesothelium is a membrane composed of simple squamous epithelial cells of mesodermal origin, which forms the lining of several body cavities: the pleura (pleural cavity around the lungs), peritoneum (abdominopelvic cavity including the mesent ...

cells from the delicate strands of connective tissue called the

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.neuroglia Glia, also called glial cells (gliocytes) or neuroglia, are non- neuronal cells in the central nervous system (the brain and the spinal cord) and in the peripheral nervous system that do not produce electrical impulses. The neuroglia make up ...

elements. Although the pia mater is primarily structurally similar throughout, it spans both the spinal cord's neural tissue and runs down the fissures of the

cerebral cortex The cerebral cortex, also known as the cerebral mantle, is the outer layer of neural tissue of the cerebrum of the brain in humans and other mammals. It is the largest site of Neuron, neural integration in the central nervous system, and plays ...

in the brain. It is often broken down into two categories, the cranial pia mater (pia mater encephali) and the spinal pia mater (pia mater spinalis).

Cranial pia mater

The section of the pia mater enveloping the brain is known as the cranial pia mater. It is anchored to the brain by the processes of

astrocyte Astrocytes (from Ancient Greek , , "star" and , , "cavity", "cell"), also known collectively as astroglia, are characteristic star-shaped glial cells in the brain and spinal cord. They perform many functions, including biochemical control of en ...

s, which are

glial cell Glia, also called glial cells (gliocytes) or neuroglia, are non-neuronal cells in the central nervous system (the brain and the spinal cord) and in the peripheral nervous system that do not produce electrical impulses. The neuroglia make up ...

s responsible for many functions, including maintenance of the

extracellular This glossary of biology terms is a list of definitions of fundamental terms and concepts used in biology, the study of life and of living organisms. It is intended as introductory material for novices; for more specific and technical definitions ...

space. The cranial pia mater joins with the

ependyma The ependyma is the thin neuroepithelial ( simple columnar ciliated epithelium) lining of the ventricular system of the brain and the central canal of the spinal cord. The ependyma is one of the four types of neuroglia in the central nervous s ...

, which lines the

cerebral ventricles 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 ventricu ...

to form

es that produce

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 ...

. Together with the other meningeal layers, the function of the pia mater is to protect the central nervous system by containing the cerebrospinal fluid, which cushions the brain and spine. The cranial pia mater covers the surface of the brain. This layer goes in between the cerebral gyri and cerebellar laminae, folding inward to create the tela chorioidea of the third ventricle and the choroid plexuses of the

lateral Lateral is a geometric term of location which may also refer to: Biology and healthcare * Lateral (anatomy), a term of location meaning "towards the side" * Lateral cricoarytenoid muscle, an intrinsic muscle of the larynx * Lateral release ( ...

and

third ventricle The third ventricle is one of the four connected cerebral ventricles of the ventricular system within the mammalian brain. It is a slit-like cavity formed in the diencephalon between the two thalami, in the midline between the right and lef ...

s. At the level of the

cerebellum The cerebellum (: cerebella or cerebellums; 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 it or eve ...

, the pia mater membrane is more fragile due to the length of blood vessels as well as decreased connection to the

Spinal pia mater

The spinal pia mater closely follows and encloses the curves of the

, and is attached to it through a connection to the anterior fissure. The pia mater attaches to the dura mater through 21 pairs of

denticulate ligaments Denticulate ligaments are lateral projections of the spinal pia mater forming triangular-shaped ligaments that anchor the spinal cord along its length to the dura mater on each side. There are usually 21 denticulate ligaments on each side, with t ...

that pass through the

and dura mater of the spinal cord. These denticular ligaments help to anchor the spinal cord and prevent side to side movement, providing stability. The membrane in this area is much thicker than the cranial pia mater, due to the two-layer composition of the pia membrane. The outer layer, which is made up of mostly connective tissue, is responsible for this thickness. Between the two layers are spaces which exchange information with the subarachnoid cavity as well as blood vessels. At the point where the pia mater reaches the

conus medullaris The conus medullaris (Latin for "medullary cone") or conus terminalis is the tapered, lower end of the spinal cord. It occurs near lumbar vertebral levels 1 (L1) and 2 (L2), occasionally lower. The upper end of the conus medullaris is usually no ...

or medullary cone at the end of the spinal cord, the membrane extends as a thin filament called the

filum terminale The filum terminale ('terminal thread') is a delicate strand of fibrous tissue, about 20 cm in length, extending inferiorly from the apex of the conus medullaris to attach onto the coccyx. The filum terminale acts to anchor the spinal cord ...

or terminal filum, contained within the lumbar

cistern A cistern (; , ; ) is a waterproof receptacle for holding liquids, usually water. Cisterns are often built to catch and store rainwater. To prevent leakage, the interior of the cistern is often lined with hydraulic plaster. Cisterns are disti ...

. This filament eventually blends with the dura mater and extends as far as the

coccyx The coccyx (: coccyges or coccyxes), commonly referred to as the tailbone, is the final segment of the vertebral column in all apes, and analogous structures in certain other mammals such as horse anatomy, horses. In tailless primates (e.g. hum ...

, or tailbone. It then fuses with the

periosteum The periosteum is a membrane that covers the outer surface of all bones, except at the articular surfaces (i.e. the parts within a joint space) of long bones. (At the joints of long bones the bone's outer surface is lined with "articular cartila ...

, a membrane found at the surface of all bones, and forms the coccygeal ligament. There it is called the central ligament and assists with movements of the trunk of the body.

Function

In conjunction with the other meningeal membranes, pia mater functions to cover and protect the

central nervous system The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain, spinal cord and retina. The CNS is so named because the brain integrates the received information and coordinates and influences the activity o ...

(CNS), to protect the blood vessels and enclose the venous sinuses near the CNS, to contain the cerebrospinal fluid (CSF) and to form partitions with the skull. The CSF, pia mater, and other layers of the meninges work together as a protection device for the brain, with the CSF often referred to as the fourth layer of the meninges.

CSF production and circulation

Cerebrospinal fluid is circulated through the ventricles,

cisterns A cistern (; , ; ) is a waterproof receptacle for holding liquids, usually water. Cisterns are often built to catch and store rainwater. To prevent leakage, the interior of the cistern is often lined with hydraulic plaster. Cisterns are disti ...

, and subarachnoid space within the brain and spinal cord. About 150 mL of CSF is always in circulation, constantly being recycled through the daily production of nearly 500 mL of fluid. The CSF is primarily secreted by the

; however, about one-third of the CSF is secreted by pia mater and the other ventricular ependymal surfaces (the thin epithelial membrane lining the brain and

central canal The central canal (also known as spinal foramen or ependymal canal) is the cerebrospinal fluid-filled space that runs through the spinal cord. The central canal lies below and is connected to the ventricular system of the brain, from which it r ...

) and arachnoidal membranes. The CSF travels from the ventricles and cerebellum through three foramina in the brain, emptying into the cerebrum, and ending its cycle in the venous blood via structures like the

arachnoid granulations 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 d ...

. The pia spans every surface crevice of the brain other than the foramina to allow the circulation of CSF to continue.

Perivascular spaces

Pia mater allows for the formation of perivascular spaces that help serve as the brain's lymphatic system. Blood vessels that penetrate the brain first pass across the surface and then go inwards toward the brain. This direction of flow leads to a layer of the pia mater being carried inwards and loosely adhering to the vessels, leading to the production of a space, namely a perivascular space, between the pia mater and each blood vessel. This is critical because the brain lacks a true lymphatic system. In the remainder of the body, small amounts of protein are able to leak from the parenchymal capillaries through the lymphatic system. In the brain, this ends up in the

interstitial space An interstitial space is an intermediate space located between regular-use floors, commonly located in Hospital, hospitals and laboratory-type buildings to allow space for the mechanical systems of the building. By providing this space, laborator ...

. The protein portions are able to leave through the very permeable pia mater and enter the subarachnoid space in order to flow in the cerebrospinal fluid (CSF), eventually ending up in the cerebral veins. The pia mater serves to create these perivascular spaces to allow passage of certain material, such as fluids, proteins, and even extraneous particulate matter such as dead white blood cells from the blood stream to the CSF, and essentially the brain.

Permeability

Due to the pia mater's and the ependyma's high permeability, water and small molecules in the CSF are able to enter the brain interstitial fluid, so the interstitial brain fluid and the CSF are very similar in terms of composition. However, regulation of this permeability is achieved through the abundant number of astrocyte endfeet processes which are responsible for connecting the capillaries and the pia mater in a way that helps limit the amount of free diffusion going into the CNS. The function of the pia mater is more simply visualized through these ordinary occurrences. This last property is evident in cases of head injury. When the head comes into contact with another object, the brain is protected from the skull due to the similarity in density between these two fluids so that the brain does not simply smash through into the skull, but rather its movement is slowed and stopped by the viscous ability of this fluid. The contrast in permeability between the pia mater and the

blood–brain barrier The blood–brain barrier (BBB) is a highly selective semipermeable membrane, semipermeable border of endothelium, endothelial cells that regulates the transfer of solutes and chemicals between the circulatory system and the central nervous system ...

means that many drugs that enter the blood stream cannot enter the brain, but instead must be administered into the cerebrospinal fluid.

Spinal cord compression

The pia mater also functions to deal with the deformation of the spinal cord under compression. Due to the high elastic modulus of the pia mater, it is able to provide a constraint on the surface of the spinal cord. This constraint stops the elongation of the spinal cord, as well as providing a high strain energy. This high strain energy is useful and responsible for the restoration of the spinal cord to its original shape following a period of decompression.

Sensory

Ventral root afferents are unmyelinated sensory axons located within the pia mater. These ventral root afferents relay sensory information from the pia mater and allow for the transmission of pain from disc herniation and other spinal injury.

Evolution

The significant increase in the size of the cerebral hemisphere through evolution has been made possible in part through the evolution of the vascular pia mater, which allows nutrient blood vessels to penetrate deep into the intertwined cerebral matter, providing the necessary nutrients in this larger neural mass. Throughout the course of life on earth, the nervous system of animals has continued to evolve to a more compact and increased organization of neurons and other nervous system cells. This process is most evident in vertebrates and especially mammals in which the increased size of the brain is generally condensed into a smaller space through the presence of sulci or fissures on the surface of the hemisphere divided into gyri allowing more superficies of the cortical grey matter to exist. The development of the meninges and the existence of a defined pia mater was first noted in the vertebrates, and has been more and more significant membrane in the brains of mammals with larger brains.Encyclopaedia Britannica
additional text.

Pathology

Meningitis is the inflammation of the pia and arachnoid mater. This is often due to bacteria that have entered the subarachnoid space, but can also be caused by viruses, fungi, as well as non-infectious causes such as certain drugs. It is believed that bacterial meningitis is caused by bacteria that enter the central nervous system through the blood stream. The molecular tools these pathogens would require to cross the meningeal layers and the

are not yet well understood. Inside the subarachnoid, bacteria replicate and cause inflammation from released toxins such as

hydrogen peroxide Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscosity, viscous than Properties of water, water. It is used as an oxidizer, bleaching agent, and antiseptic, usua ...

(H₂O₂) . These toxins have been found to damage the

mitochondria A mitochondrion () is an organelle found in the cells of most eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is us ...

and produce a large scale immune response. Headache and meningismus are often signs of inflammation relayed via trigeminal sensory nerve fibers within the pia mater. Disabling neuropsychological effects are seen in up to half of bacterial meningitis survivors. Research into how bacteria invade and enter the meningeal layers is the next step in prevention of the progression of meningitis. A tumor growing from the meninges is referred to as a

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 ...

. Most meningiomas grow from the arachnoid mater inward applying pressure on the pia mater and therefore the brain or spinal cord. While meningiomas make up 20% of primary brain tumors and 12% of spinal cord tumors, 90% of these tumors are benign. Meningiomas tend to grow slowly and therefore symptoms may arise years after initial tumor formation. The symptoms often include headaches and seizures due to the force the tumor creates on sensory receptors. The treatments available for these tumors include surgery and radiation.

Additional images

File:Meninges-en.svg, Meninges of the CNS File:Brain Dissection - Pia Mater and Derivatives - Sanjoy Sanyal.webm, Human brain dissection. Demonstrating removal of pia mater from the left

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 ...

. File:Gray720.png, Median sagittal section of brain File:Gray749.png, Coronal section of inferior horn of lateral ventricle File:Gray769-en.svg, Diagrammatic representation of a section across the top of the skull, showing the membranes of the brain, etc. File:Gray1196.png, Diagrammatic section of scalp File:Viorel Pais - Ultrastructural diagram of the cerebral cortex.jpg, Ultrastructural diagram of the cerebral cortex ( Viorel Pais, 2012)