Scientists have long believed that the processes required for embryonic development are recapitulated during the regeneration of damaged body parts, such as fractured bones. In a new study published in Development, USC Stem Cell researchers Sandeep Paul, Simone Schindler and colleagues use the regeneration of the zebrafish jawbone to show that this is not necessarily true.

During the regeneration of the zebrafish’s lower jawbone, cartilage tissue initially bridges the fracture or gap. In the cells of this repair cartilage, known as chondrocytes, genes become activated that are normally associated with bone-forming cells called osteoblasts. These genes enable the chondrocytes to produce the hard calcium phosphate crystals that form bone during repair — a task mainly assigned to osteoblasts during embryonic development.

The researchers found that the unusual bone-producing repair chondrocytes originate from a thin lining of stem cells surrounding the surface of bones. Normally, these stem cells would help maintain bone mass throughout life. However, upon injury, the stem cells are instead instructed to make cartilage that helps heal the bone. A critical signal that causes stem cells to shift from making bone to cartilage during repair is a gene called indian hedgehog a (ihha). Accordingly, zebrafish lacking ihha are unable to make cartilage in response to bone injury, and they heal poorly.

To read more, visit stemcell.usc.edu/?p=4230.