Muscle Cells Found to Help Repair Broken Bones

A study in Bone Research identifies dormant progenitor cells in skeletal muscle that move to fracture sites and turn into bone-forming cells. The findings could point to new ways to speed healing after fractures and reduce abnormal bone growth after injury.

Why it matters: - The study suggests skeletal muscle is not just a support tissue. It can supply cells that help rebuild broken bone. - The same cell population also appears to drive heterotopic ossification, or bone growth in soft tissue, after injury. - That dual role makes the cells a potential target for therapies that could both improve fracture repair and limit unwanted bone formation.

What happened: - Researchers published a study online July 6, 2026, in Bone Research showing that fibroadipogenic progenitors, or FAPs, from skeletal muscle can become bone-forming cells after fracture. - The work was led by Dr. Ugur M. Ayturk and colleagues from the Skeletal Health and Orthopedic Research Program in the U.S. - The study also found a smaller contribution from superficial periosteal cells, which sit in the connective tissue covering bone.

The details: - The team identified Clec3b as a specific marker for these dormant progenitor cells. - Researchers used a mouse model that fluorescently labeled Clec3b-expressing cells to track where the cells went before and after injury. - Under normal conditions, the Clec3b-lineage cells stayed in skeletal muscle and the superficial periosteum. - The cells did not enter bone or become osteoblasts during normal bone growth. - After fracture, the cells rapidly moved to the injury site. - Many of the cells then differentiated into osteoblasts that built new bone. - Within three weeks, about 28% of the osteoblasts in the healing callus came from Clec3b-lineage cells. - Some of the cells also became bone marrow stromal cells, which help rebuild the bone’s internal support environment. - The cells stopped expressing Clec3b as they differentiated, linking the marker to the dormant progenitor state. - Single-cell RNA sequencing confirmed that Clec3b-lineage cells gave rise to populations with the molecular features of osteoblasts and bone marrow stromal cells after fracture. - The experiments showed skeletal muscle was the main source of these regenerative cells. - Even after the periosteum was surgically removed before injury, Clec3b-positive cells still reached the fracture site and became bone-forming cells. - Bone grafts that included surrounding muscle produced substantially more Clec3b-lineage bone-forming cells than grafts without muscle. - In mouse models of heterotopic ossification, Clec3b-lineage cells also differentiated into cartilage- and bone-forming cells and became a major source of the abnormal bone. - Blocking a key bone-formation pathway or depleting the cells significantly reduced both fracture healing and abnormal bone growth, Dr. Ayturk said. - The paper was titled “Clec3b+ extraskeletal cells regulate fracture healing and heterotopic ossification.” - The journal listed the DOI as 10.1038/s41413-026-00532-6.

Between the lines: - The findings broaden the view of bone repair beyond cells already inside the skeleton. - The study points to a shared cellular program behind normal healing and harmful bone formation after trauma. - That overlap matters because a future therapy may need to boost these cells in one setting while restraining them in another. - The research also strengthens the idea that the muscle environment can serve as a reservoir of repair cells.

What's next: - The researchers believe these cells could become therapeutic targets for both fracture healing and prevention of heterotopic ossification. - Future work will likely focus on how to activate the cells safely to speed repair without triggering extra bone growth. - The publication notes the study was supported by grants from the S&L Marx Foundation, Giuliani Foundation, and the Stavros Niarchos Complex Joint Reconstruction Center at Hospital for Special Surgery, plus fellowship support from the Francis Glorieux Research Fellowship from the Canadian Osteogenesis Imperfecta Society. - Bone Research is available at the journal website. - The editorial office also listed Bone Research on X.