The molecular and cellular mechanisms involved in bone development provide an insight into the nature of bone regeneration. Sox9 is a key transcription factor for chondrogenesis and is also expressed in osteochondroprogenitors during embryonic bone development. However, it has not been determined whether Sox9-expressing cells appear during fracture repair other than in the cartilaginous callus. On the other hand, the difference between bone development and repair is that the motion of the fractured segments is associated with the subsequent fate decision of osteochondrogenic precursors between osteogenesis or chondrogenesis, but the underlying mechanism of this still has to be elucidated. We herein evaluate whether Sox9-expressing cells appear during osseous regeneration in the initial stages of fracture healing in vivo. We also investigated the association between Sox9 induction and mechanical stress and the role of Runx1 expression. As a result, Sox9-and Runx1-expressing cells were detected in the periosteal callus together with Runx2 expression. Their expression levels were significantly downregulated during its ossification, as observed in embryonic bone development. The application of cyclic tension to isolated and cultured stromal cells resulted in the upregulation and maintenance of Sox9 mRNA expression in vitro. These results showed that as in early skeletal development, Sox9-and Runx1-expressing precursor cells first appear in the periosteal callus as an early fracture repair response. Our findings also suggested that the mechanical environment modulates Sox9 expression levels in osteochondrogenic precursors and consequently influences their fate decision between osteogenic and chondrogenic lineage commitment.
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