Trans differentiation of Chondrocytes to Osteoblasts during Endochondral Ossification in the Healing Mandible

A new model of endochondral ossification has recently emerged, in which chondrocytes of the cartilage intermediate undergo a transient stem cell‐like state and ultimately trans differentiate into osteoblasts/cytes that form the new bone. This model has been established by studying long bones during development and fracture healing. However, it has yet to be determined if the same mechanism translates to mandible fracture healing. Contrary to long bones, the mandible develops primarily through intramembranous ossification; however, it heals through both intramembranous and endochondral ossification. We sought to determine whether fracture stability directs the relative contribution of each ossification mechanism during mandible healing and to determine whether chondrocytes $trans differentiate into osteoblasts/cytes during mandibular endochondral fracture healing. We hypothesized that stable mandible fractures heal through intramembranous ossification and unstable fractures heal through endochondral ossification, in a manner analogous to appendicular fractures, and that chondrocytes trans differentiate into osteoblast/cytes during endochondral ossification. To test this, we generated mandible fractures with different levels of stability by creating either 1mm trephine defects or unstable transverse fractures in the mandibular rami of C57BL/6 mice and compared the ossification process at multiple time points using histology and immunohistochemistry. Preliminary data demonstrate minimal cartilage formation in stable fractures compared to the robust cartilage formation in unstable fractures, supporting our hypothesis that mobility promotes endochondral ossification. Our data illustrate maximum cartilage formation in unstable fractures at 10 days post‐fracture. At this time point, hypertrophic chondrocytes at the chondrosseous junction, termed the “transition zone,” stained positive for the pluripotency factors Oct4 and Sox2 by immunohistochemistry. These data parallel our studies of tibia fracture repair where we have performed genetic lineage tracing experiments with inducible Col2 , Oct4 , and Sox2 reporter mice. Taken together these data suggest that motion during mandible fracture repair promotes endochondral ossification and that chondrocytes trans differentiate into osteoblasts by acquiring what appears to be a transient pluripotent state. Future studies will elucidate the mechanisms that regulate chondrocyte trans differentiation during mandible repair and test novel therapeutic strategies designed to treat hypertrophic non‐unions. Support or Funding Information NIH: NIAMS: F32AR062469 to C.B., R01‐AR057344 to T.M., NIA: R01‐AG046282 to R.M. MTF Junior Investigator Award to C.B.