Spinal pia mater

The spinal pia mater (or pia mater spinalis) is the innermost layer of the spinal meninges. In congruence to the cranial pia being closely related to the surface of the brain, the spinal pia is closely related to the surface of the spinal cord.

Gross anatomy

The spinal pia mater is continuous with the cranial pia mater at the foramen magnum . It is closely adherent to the spinal cord, enclosing it until the level of the conus medullaris . The pia mater is attached to the spinal cord at the anterior median fissure and is separated from the arachnoid mater by the subarachnoid space, which is filled with CSF .

Like the cranial pia, the spinal pia is highly vascular . It invaginates the anterior spinal artery and posterior spinal arteries, and covers the vasa corona .

The pia also extends over the exiting spinal nerve roots and blends with their epineurium .

Denticulate ligaments

The pia has numerous outward projections, the denticulate ligaments, that pierce through the arachnoid mater and attach to the outer dural layer . These have the function of suspending the spinal cord relatively centrally within the spinal canal, which is particularly important during movement of the vertebral column. There are 21 pairs of triangular-shaped outward projections that extend laterally from the pia, with their apices attaching to the outer dura . They are referred to as the denticulate ligaments of the spinal cord due to their teeth-like appearance .

Filum terminale

A further thread-like inferior extension of the pia, the filum terminale, extends from the apex of the conus medullaris to attach to the posterior surface of the S2 vertebral body (piercing through the theca) . The filum terminale can be up to 20 cm long and is surrounded by the spinal nerves that form the cauda equina in the lumbar cistern . It is enclosed by a thin layer of dura mater that is continuous from the point at which it pierces the theca and provides longitudinal support and stability to the spinal cord .

Whilst the pial component of the filum terminale extends to the S2 vertebral level, the thin dural sheath covering it ­extends further inferiorly to the level of the first segment of the coccyx - this portion of the structure is often referred to as the coccygeal ligament .