Structure
Pia mater is the thin, translucent, mesh-like meningeal envelope, spanning nearly the entire surface of the brain. It is only absent at the natural openings between the ventricles, the foramen of Majendie and the foramina of Luschka. The pia is firmly adhered to the surface of the brain and loosely connected 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 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).
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