The disks have an outer ring of fibrocartilage, the anulus fibrosus, and within it, a gelatinous material called the nucleus pulposus (Fig. 8).
The organic matrix characteristic of each zone is distinct. In the anulus fibrosus, there is an outer layer of approximately 90 concentric lamellae of type I collagen fibers; deep within that is a region of a less dense, type II collagenous matrix. This inner zone is less organized than the outer zone of the anulus fibrosus. The most superficial an- terior fibers of the anulus fibrosus blend with the anterior longitudinal ligament, whereas the most superficial posterior fibers blend with the posterior anterior longitudinal ligament. Protrusion of the nucleus pulposus through tears in the anulus fibrosus is called a herniated disk. Because the anterior anulus fibrosus is thicker than its posterior counter- parts, posterior herniations are more common. When this herniation compresses a spi- nal nerve root, pain that radiates down the leg (known as sciatica) can result.
In contrast to the collagen-rich anulus fi- brosus, the nucleus pulposus is predominantly proteoglycan (Fig. 9).
It is the interaction between large proteoglycan molecules and water that gives the nucleus pulposus its re- sistance to compression. The cells populat- ing the anulus fibrosus are like fibroblasts, while those in the nucleus pulposus are more chondrocytic in appearance and have synthetic characteristics.