The disks have an outer ring of fibrocartilage, the anulus fibrosus, and within it, a gelatinous material called the nucleus pulposus (Fig. 8).

Figure 8
Sagittal section view of two vertebral bodies and an intervertebral disk. The three regions of the disk are shown: cartilaginous end plate, outer anulus fibrosus, inner anulus fibrosus, and nucleus pulposus. The pos- terior articular and spinous processes and the articular surface of a facet joint are also shown.
(Reproduced with permission from Ashton-Miller JA, Schultz AB: Biomechanics of the human spine, in Mow VC, Hayes WC (eds): Basic Orthopaedic Biomechanics. Philadelphia, PA, Lippincott-Raven, 1997, pp 353-393.)

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).

Micrographs showing the arrangement of the collagen fibrils in the outer anulus fibrosus (A) and the central nucleus pulposus (B). Note the tightly packed, highly oriented lamellae of collagen fibrils in the outer anulus fibrosus and the loose, almost random pattern of collagen fibrils in the nucleus pulposus.
(Reproduced with permission from Buckwalter JA: The fine structure of human intervertebral disc, in White AA III, Gordon SL (eds): American Academy of Orthopaedic Surgeons Symposium on Idiopathic Low Back Pain. St. Louis, MO, CV Mosby, 1982, pp 108-143.)

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.