Cartilage remnants (purple) disrupt bones (pink) in MT1-MMP–deficient mice (right).
In the well-known pathway to bone formation, a cartilage scaffold must be mineralized before it is degraded by osteoclasts and replaced with bone. But some bones seem to be formed without cartilage mineralization. Holmbeck et al. find that this process relies on the matrix metalloprotease MT1-MMP, which degrades unmineralized cartilage.
The authors examined bone formation in MT1-MMP–deficient mice, which have skulls that are misshapen by cartilage. They find that in wild-type mice this same cartilage is not mineralized, but rather expresses MT1-MMP before its removal and replacement with bone. Without the protease, the cartilage remains, and adult bone layers do not form correctly....
