According to the New York University Langone Medical Center, intervertebral discs are made up of "fibrocartilage tissue," which has two layers. While the inner portion resembles jelly, the outermost part has strong and fibrous layers. About 90 percent of this tissue is water.
NYLMC explains that intervertebral discs lose water over time, which causes them to compact. This is why people become slightly shorter as they age. The shrinkage also makes discs stick out slightly. If a disc's outer layer breaks down and lets the soft inner core emerge, it results in a herniated or "slipped" disc and extra pressure is put on the spinal cord or spinal nerve roots. This irritation often leads to pain and back or leg weakness.