[5] Pia mater is the thin, translucent, mesh-like meningeal envelope, spanning nearly the entire surface of the brain.

A subarachnoid space exists between the arachnoid layer and the pia, into which the choroid plexus releases and maintains the cerebrospinal fluid (CSF).

[7] 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.

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.

[9] 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 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.

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.

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.

[12] Pia mater allows for the formation of perivascular spaces that help serve as the brain's lymphatic system.

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.

[3] 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.

Due to the high elastic modulus of the pia mater, it is able to provide a constraint on the surface of the spinal cord.

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.

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.

Headache and meningismus are often signs of inflammation relayed via trigeminal sensory nerve fibers within the pia mater.