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Other Terms: Intervertebral disc, Discus intervertebralis, Disques intervertébraux
The annulus consists of consecutive rings of collagen fibers, called lamellae, arranged in a highly ordered pattern. Anywhere from ten to twenty lamellae are present in the annulus. They resemble the growth rings of a tree and surround the central nucleus pulposus. The annulus is formed of strong fibrocartilage. The collagen fibers in each lamellae alternate, one to the right the next to the left, but their orientation with respect to the vertical is always approximately 65 to 70 degrees. The lamellae are thicker in the anterior and lateral portions of the disc and thin posteriorly. In the young disc, the nucleus pulposus is a semi-fluid mucoid mass that is the remnant of the embryological notochord. It has a few scattered cartilage cells and collagen fibers. It is in essence a noncompressible fluid. It is approximately 70 to 90% water with proteoglycans forming the next major constituent. The bottle brush proteoglycans trap and hold the water within their domains. Dispersed collagen fibers make up the final major component. With increasing age, the nucleus pulposus becomes more cartilaginous and less fluid-like.
The disc allows movement between the vertebral bodies, absorbs shock, and transmits loads from one vertebra to the next.