Chondrocytes Directly Transform into Bone Cells in Mandibular Condyle Growth

Area of Science:

• Skeletal Biology
• Developmental Biology
• Cell Biology

Background:

• The traditional view posits hypertrophic chondrocytes undergo apoptosis before endochondral bone formation.
• Recent long bone studies suggest chondrocytes may directly differentiate into bone cells.
• Mandibular condylar cartilage presents a unique model for studying endochondral ossification.

Purpose of the Study:

• To investigate the fate of hypertrophic chondrocytes in mandibular condylar cartilage.
• To determine if chondrocytes directly transform into bone cells in this specific anatomical site.
• To challenge the long-held apoptosis-centric model of endochondral ossification.

Main Methods:

• In vivo characterization of condylar hypertrophic chondrocytes.
• Ex vivo culture of condylar cartilage explants.
• Cartilage-specific cell lineage tracing using genetically engineered mice (Rosa 26(tdTomato), 2.3 Col1(GFP), aggrecan Cre(ERT2), and Col10-Cre).

Main Results:

• Condylar hypertrophic chondrocytes exhibit antiapoptotic markers (Bcl-2), proliferation, and early bone markers (alkaline phosphatase).
• Ex vivo cultures show peripheral cells expressing a bone marker (Col1).
• Lineage tracing confirms direct in vivo transformation of chondrocytes into bone cells, originating from the inferior condylar cartilage.

Conclusions:

• A majority of chondrocytes in mandibular condylar cartilage directly transform into bone cells.
• This transformation process contributes significantly to bone formation in the condyle.
• Ossification initiates at the inferior aspect of the mandibular condylar cartilage, expanding unidirectionally.