Messenger cells bring good news for bone healing

How do bones heal, and how could they heal better? The answer to these questions may lie in a newly discovered population of “messenger” cells, according to a recent USC Stem Cell study published in the journal eLife.

“With nearly half a million patients in the U.S experiencing failed bone repair every year, stimulating these ‘messengers’ and other key cell types could accelerate repair and prevent non-unions,” said corresponding author Francesca Mariani, an associate professor of stem cell biology and regenerative medicine, and integrative anatomical sciences at the Keck School of Medicine of USC.

In their study, first author Stephanie T. Kuwahara and her colleagues looked at the mammalian rib bone, which regenerates uncommonly well. In fact, orthopaedic surgeons have noted that missing sections of the rib as long as eight inches will readily grow back in patients.

By observing similar rib surgeries in mice, the scientists proposed a model for how mammals repair large bone injuries, and identified key cell types essential to the healing process.

One of these key cell types is a small population of “messenger” cells, which can be identified by the activity of a gene called Sox9. These messenger cells reside in the sheath of tissue, called the periosteum, which surrounds each rib. Upon injury, a protein called “Hedgehog” activates the messenger cells, which in turn tell neighboring cells to differentiate into a hybrid between cartilage and bone. These hybrid cells form the “repair callus” that converts into newly regenerating bone.