In children, bone grow thicker and longer and get stronger and denser. When children reach adolescence, they know that the time has come to stop growing longer and stronger. However, even into adulthood, bones still retain the capacity to heal. Why the differences between adolescents and adults? This is a question that has long fed the imaginations of scientists.
Recently, a collaborative team of biomedical researchers from the University of Michigan, Kyoto University and Harvard University has made the answer to this question a little clearer.
Dr. Noriaki Ono, U-M assistant professor of dentistry, and his collaborators discovered that a certain subset of cartilage-making cells – cells known as chondrocytes – proliferate and differentiate into other bone cells that drive bone growth. These discoveries could lead to new treatments for children with facial deformities who normally have to wait until adulthood for corrective surgery. This study appeared in the journal Nature Cell Biology.
A long-held view is that bone-making chondrocytes die once children reach adolescence and their bones stop growing. However, in adults, bone still heal without the benefit of these bone-making chondrocytes. How does this occur? This question has generated a fair amount of disagreement between researchers.
Ono’s group discovered that some of these bone-making chondrocytes don’t die. Instead, they are transformed into other types of bone-growing and bone-healing cells.
“Up until now, the cells that drive this bone growth have not been understood very well. As an orthodontist myself, I have special interest in this aspect, especially for finding a cure for severe bone deformities of the face in children,” he said. “If we can find a way to make bones that continue to grow along with the child, maybe we would be able to put these pieces of growing bones back into children and make their faces look much better than they do.”
According to Ono, one of the challenges in bone and cartilage medicine is that resident stem cells haven’t really been identified. The only widely accepted idea is that certain stem cells like mesenchymal stem cells help bones heal and other help them grow, but the progenitor cells for these cell populations and what goes wrong with them in conditions such as osteoporosis remains mysterious.
Ono and his team used a technique called “fate mapping,” which labels cells genetically and them follows them throughout development. Ono and others came upon a specific precursor cell that gives rise to fetal chondrocytes, and all the other later bone-making cells and . By mesenchymal stromal cells later in life. Most exciting, Ono and his coworkers found a way to identify the cells responsible for growing bone. By identifying these cells, isolating them and even implanting them into the skull or long bones of a child with a bone deformity condition, the cells would make bone that would also grow with the child.
Many factors cause craniofacial deformities. These types of deformities can place pressure on the brain, eyes, or other structures and prevent them from developing normally. For example, children with Goldenhar syndrome have underdeveloped facial tissues that can harm the developing jawbone. Another bone deformity called deformational plagiocephaly causes a child’s head to grow asymmetrically. Maybe the implantation of such cells can provide a way to restart the abnormally growing bones in these children.