New study identifies how stem cells mobilize to mend a fracture

Much like EMTs rushing to the scene after an accident, stem cells hurry to the site of a skull fracture to start mending the damage. A new finding has uncovered the signaling mechanism that triggers their response.

October 21, 2025

When a head injury results in a skull fracture, stem cells—the cells reserved to replace dead cells or repair damaged cells—rush to the site of the injury and start multiplying within 24 to 48 hours after the injury occurs. What hasn’t been fully known until now is how the cells know where and when to move, to multiply, and to make up for the lost cells. That is the subject of a new finding recently published in Science Signaling by Dr. Yingzi Yang, professor of Developmental Biology at Harvard School of Dental Medicine (HSDM).

Diagram illustrating how suture stem cells (SuSCs) respond to skull injury. The three stages shown are: (I) SuSC expansion in the suture, regulated by Shh/Ihh signaling; (II) cell migration toward the calvarial bone guided by Cxcl12 signaling; and (III) cell proliferation and osteoblast differentiation at the injury site, supported by Shh/Ihh. Arrows indicate molecular signaling and feedback loops among Shh/Ihh and Cxcl12, which coordinate stem cell movement, multiplication, and bone regeneration. — *Schematic illustration of the Gli1+ SuSCs activation, directional migration,* *proliferation and osteoblast differentiation in response to cranial bone injury.*

Yang and her team of researchers specifically studied craniofacial bones to better understand stem cell activation, particularly stem cell migration and expansion, during tissue repair to identify signaling molecules that tell the stem cells where to go and how to expand.

“We looked at how these stem cells respond to injury, expand in number, migrate to the injury site, and differentiate into cells to regenerate lost tissue. Suture stem cells (SuSCs) are important for homeostasis and regeneration of cranial bone and can be used as a model to understand stem cell regulation for tissue regeneration at a distance,” said Yang.

The researchers found that after injury, the signaling molecules Cxcl12, Shh, and Ihh work together to help Gli1+ stem cells multiply in number. They also guide the stem cells and their descendants to migrate specifically to the damaged area and develop into bone-forming cells.

“Our findings represent a major step forward to allow us not only to harness the capacity of the SuSCs for cranial bone regeneration, but also to pinpoint the cell of origin that causes craniofacial defects such as craniosynostosis, a disorder resulting from premature suture closure, in genetic diseases and other conditions,” said Yang.

The finding highlights the vital role of suture stem cells, shedding light on genetic disorders that affect skull formation and conditions such as progressive osseous heteroplasia and fibrous dysplasia. By understanding the underlying mechanisms, better treatments could be found for these debilitating conditions.

“Currently, surgery is quite often the only option for treating these conditions. However, now that we know how natural healing is controlled, our hope is that it could open the door to non-surgical approaches,” said Yang.

This work was supported by National Institutes of Health grants R01DE025866.