Pia mater

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Meninges and superficial vessels of the brain.

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.