A Novel Role of Osterix in Mouse Mandibular Condyle Formation

Objective: Mandibular condyle formation depends on the processes of chondrogenesis and osteogenesis. Osterix (Osx) is a transcription factor essential for osteoblasts differentiation and mineralization, but its function in  mandibular condyle formation is unknown. The goal of this study was to study Osx expression pattern and its function in mouse mandibular condyle development. 

Method: 1) Both Osx-lacZ knock-in, Osx-GFP:Cre/Rosa 26 mice, and immunohistochemistry were used for tracing Osx expression patterns during condyle development; 2) conditional osterix-null mice were generated by crossing osterix loxP mice to the 2.3 Col I-Cre mice; and 3) combined approaches of x-ray, micro-CT, and histology methods were used to characterize the mandibular condyle phenotypes. 

Result: 1) X-gal staining of Osx lacZ knock-in, and Osx-GFP:Cre/Rosa 26 mice showed that Osx was widely expressed in the rest cell zone, proliferation cell zone, hypertrophic cell zone and osteoblasts in condyle; 2) Conditional deletions of osterix led to a malformed  mandibular condyle, including i) decreased size of condyle head, decreased bone volume in both condyle head and neck; ii) HE and safranin O staining showed an increased width of the hypertrophic cell zone; and iii) in situ hybridization revealed an increased levels of collagen II and collagen X mRNA expression in Osx cKO condylar cartilage.

Conclusion: Osterix plays important roles in mandibular condyle formation through its dual roles in control of both osteogenesis and chondrogenesis (Work supported by NIH grant DE018486 to JQF).