MadSci Network: Medicine

Re: why do bone injuries heal much faster than cartilage

Date: Mon Feb 16 09:13:35 2004
Posted By: Paul Odgren, Assistant Professor, Dept. of Cell Biology
Area of science: Medicine
ID: 1074549675.Me

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

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