|MadSci Network: Medicine|
Dear Marcela, Excellent question. The reason that bone injuries heal faster than cartilage injuries has to do with the ability of the cells in each tissue to respond to injury by dividing and making new tissue. The cartilage that makes up the slippery, gliding surfaces of your joints, like knees, wrists, between your vertebrae, is inhabited by specialized cells called “chondrocytes.” They live within the cartilage and are pretty inactive once the bones have formed and grown to full size. They respond very little to injuries, and they hardly ever divide after growth is complete. Damage to the joint cartilage (“articular cartilage”) can be very painful and crippling, whether from an injury or from arthritis. Bone, on the other hand, is covered by cells that can divide and make new bone. The amazing thing about the process of fracture healing is the sequence of events that occurs after a fracture. To understand this, you should know that most bones in your body except your skull first form as a cartilage model of the bone that, during growth, is progressively eaten and replaced by bone. During fracture healing, the whole process goes on again in fast- forward. Cells on the surface of the bone actually turn into chondrocytes that make a kind of cartilage filler for the crack, called a “fracture callus.” Over the course of a few weeks, specialized bone cells called osteoclasts eat up the cartilage, and bone-forming cells called osteoblasts replace the whole thing with new bone. Why chondrocytes can become activated in bone healing but not in healing joint cartilage remains a mystery, although one that is being actively investigated by many scientists who are trying to find ways to get cartilage to heal. I hope this answers your question. Paul Odgren, Ph.D. Department of Cell Biology University of Massachusetts Medical School Worcester
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