Intervertebral Disc Tissues

Cartilagenous intervertebral discs separate adjacent vertebrae in the spine, allowing modest spinal movements, and distributing compressive load evenly on the vertebral bodies. A central fluid-like nucleus pulposus region is surrounded by a tough laminated annulus fibrosus. Because adult discs lack a blood or nerve supply, cell density is very low and tissue turnover is very slow. Consequently, discs have only a limited ability to heal following injury, and they exhibit a marked and progressive age-related deterioration in material properties. Changes in composition of old discs include fragmentation and loss of proteoglycans, loss of water (especially from the nucleus), and increasing cross-linking of collagens. Functionally, old disc tissues become dry, fibrous, and stiff. Pressure in the nucleus decreases, and stress concentrations grow in the annulus, which becomes more vulnerable to injury and herniation (“slipped disc”). Physical disruption allows nerves and blood vessels to grow into the tissue, which can become a source of chronic back pain. The aging annulus weakens, despite increased collagen crosslinking, because small defects accumulate in its lamellar structure, causing microscopic delamination. Progressive collapse of the annulus can trigger a “degenerative cascade” of events, including spinal osteoarthritis (as load-bearing is transferred to the neural arch), segmental instability, osteophytosis, and senile kyphosis.