Pia

The pia mater is the innermost layer of the meninges. Unlike the dura and arachnoid, the cranial pia closely follows the contours of the brain, including the grooves and folds (sulci and gyri). As the cranial pia mater extends through the foramen magnum, it becomes the spinal pia mater and tightly surrounds the spinal cord.

Histologically, the pia mater consists of two sub-layers: the outer layer called the epipal layer, which is made up of collagen fibers, and the inner intimal layer, which contains elastic and reticular fibers. The outer layer is linked to the arachnoid mater through arachnoid trabeculae, while the inner layer is in contact with the brain tissue through the astrocytic glial cells.

The cranial pia wraps around blood vessels as they enter the brain parenchyma from the subarachnoid space, forming interstitial fluid-filled perivascular spaces called Virchow-Robin spaces around the blood vessels.

There are two notable extensions of the spinal pia mater: the denticulate ligaments and the filum terminale. The denticulate ligaments are extensions of the pia mater and can be found on the sides of the spinal cord. They anchor the spinal cord by piercing through the arachnoid mater and attaching to the dura mater. These ligaments are paired and run along the entire length of the spinal cord, situated between the ventral and dorsal nerve roots.

In adults, the spinal cord ends at the L1/L2 vertebral level, but the pia mater continues downward. This downward extension is known as the filum terminale (link), which firmly attaches to the tip of the coccyx, providing vertical support for the spinal cord.