The intervertebral discs have three components that enable them to flex and twist while bearing lots of weight. In its center (white on MRI because of a high water content) is the nucleus pulpous (NP). It is white in color and resembles crab meat. The second component fixes the disc to the top and bottom of adjacent vertebrae through tough hyaline cartilage that never detaches (the black lines on edges of vertebrae on this MRI scan). It is tough semi-transparent “gristle” like that seen on the end of a piece of chicken “drumstick”. The third component is the annulus fibrosus, a ligamentous material around the outside that holds the two vertebrae in alignment and keeps the NP in the center. With aging, the NP loses some water content, narrowing the space between vertebrae. The person loses some height and on MRI the NP gets darker.
The five vertebrae between where the last rib comes off (T12) and the Sacrum (S1) are the lumbars. Thus, there are five lumbar discs, L1-2, L2-3, L3-4, L4-5, and L5-S1. Discs have no blood vessels and depend on diffusion of oxygen and nutrients from adjacent vertebrae. The annulus fibrosus contains nerves, so stretching or tearing it can cause back pain.