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Jai Ganesh

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Pia Mater

Pia Mater

Gist

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.

The pia mater is the innermost layer of the meninges and together with the arachnoid mater is referred to as the leptomeninges. It is closely related to the surface of the brain and unlike the arachnoid mater extends into the sulci

The pia mater is the innermost layer of the meninges and together with the arachnoid mater is referred to as the leptomeninges. It is closely related to the surface of the brain and unlike the arachnoid mater extends into the sulci.

Summary

Pia is the innermost layer of the meninges, the connective tissue layers that surround the central nervous system. It is a thin, transparent, delicate layer that hugs the surface of the brain and spinal cord, closely following its contours. The pia and arachnoid layers of the meninges are collectively called the leptomeninges. Located between these two layers is the cerebrospinal fluid containing subarachnoid space.

The pia is highly vascularized by a network of fine blood vessels. In the cranium, the pia extends for a short distance along the surface of blood vessels as they travel to and from the meninges to enter and exit the cerebral cortex. This pia extension is called a pial coat. Similar extensions are seen in the spinal cord where the pia follows spinal nerves and spinal blood vessels for a short distance as they travel away from the central cord. Fingers of vascularized pia covered by epithelial cells form invaginations into ventricles called choroid plexuses. Choroid plexuses function to secrete cerebrospinal fluid.

In the spinal cord, the pia forms tiny ligaments called the denticulate ligaments. These ligaments suspend the spinal cord within the dural sac. The inferior most aspect of the spinal meninges is anchored to the coccyx by a thin strand called the filum terminale. This filum has internal and external parts, the internal being formed by a strand of pia.

Details

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 (glymphatic system). When the pia mater becomes irritated and inflamed the result is meningitis.

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, and the lateral aperture. 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 releases and maintains the cerebrospinal fluid (CSF). The subarachnoid space contains trabeculae, 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, 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 covered by mesothelial cells from the delicate strands of connective tissue called the arachnoid trabeculae. In the perivascular spaces, the pia mater begins as mesothelial lining on the outer surface, but the cells then fade to be replaced by neuroglia 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 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 astrocytes, which are glial cells responsible for many functions, including maintenance of the extracellular space. The cranial pia mater joins with the ependyma, which lines the cerebral ventricles to form choroid plexuses that produce cerebrospinal fluid. 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 and third ventricles. At the level of the cerebellum, the pia mater membrane is more fragile due to the length of blood vessels as well as decreased connection to the cerebral cortex.

Spinal pia mater

The spinal pia mater closely follows and encloses the curves of the spinal cord, 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 that pass through the arachnoid mater 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 or medullary cone at the end of the spinal cord, the membrane extends as a thin filament called the filum terminale or terminal filum, contained within the lumbar cistern. This filament eventually blends with the dura mater and extends as far as the coccyx, or tailbone. It then fuses with the periosteum, 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.

Additional Information

The pia mater is the delicate innermost layer of the meninges, directly enveloping the brain and spinal cord and adhering closely to the contours of the central nervous system (CNS) surface, including the gyri and sulci of the brain and the external surfaces of the spinal cord. It is composed of connective tissue, including collagenous, reticular, and elastic fibers, and contains numerous blood vessels that penetrate the CNS parenchyma. The pia mater is covered superficially by a single layer of flattened mesothelial cells and is anchored to the underlying cortex by a glial membrane.

This meningeal layer plays a significant role in maintaining the neural microenvironment through its involvement in neurovascular interactions. It forms sheaths around blood vessels entering and exiting the CNS, creating perivascular spaces known as Virchow–Robin spaces. These spaces are important for cerebrospinal fluid (CSF) absorption and clearance and are thought to communicate with the subpial space. The pia mater is continuous with the glia limitans, a thin subpial layer comprised of astrocyte endfeet, which insulates the CNS from substances present in the vascular and subarachnoid compartments. The subarachnoid space, located between the arachnoid mater and the pia mater, contains CSF, which circulates and provides mechanical protection, nutrient delivery, and waste removal for the CNS.

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