Abstract
For decades, it has been widely accepted that hypertrophic chondrocytes undergo apoptosis prior to endochondral bone formation. However, very recent studies in long bone suggest that chondrocytes can directly transform into bone cells. Our initial in vivo characterization of condylar hypertrophic chondrocytes revealed modest numbers of apoptotic cells but high levels of antiapoptotic Bcl-2 expression, some dividing cells, and clear alkaline phosphatase activity (early bone marker). Ex vivo culture of newborn condylar cartilage on a chick chorioallantoic membrane showed that after 5 d the cells on the periphery of the explants had begun to express Col1 (bone marker). The cartilage-specific cell lineage-tracing approach in triple mice containing Rosa 26(tdTomato) (tracing marker), 2.3 Col1(GFP) (bone cell marker), and aggrecan Cre(ERT2) (onetime tamoxifen induced) or Col10-Cre (activated from E14.5 throughout adult stage) demonstrated the direct transformation of chondrocytes into bone cells in vivo. This transformation was initiated at the inferior portion of the condylar cartilage, in contrast to the initial ossification site in long bone, which is in the center. Quantitative data from the Col10-Cre compound mice showed that hypertrophic chondrocytes contributed to ~80% of bone cells in subchondral bone, ~70% in a somewhat more inferior region, and ~40% in the most inferior part of the condylar neck (n = 4, P < 0.01 for differences among regions). This multipronged approach clearly demonstrates that a majority of chondrocytes in the fibrocartilaginous condylar cartilage, similar to hyaline cartilage in long bones, directly transform into bone cells during endochondral bone formation. Moreover, ossification is initiated from the inferior portion of mandibular condylar cartilage with expansion in one direction.