Chondrocytes Directly Transform into Bone Cells in Mandibular Condyle Growth

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.

Roles of Chondrocytes in Endochondral Bone Formation and Fracture Repair

“…While apoptosis of hypertrophic chondrocytes surely takes place, we have shown that apoptotic chondrocytes in the deepest layers of the MCC are relatively few (Jing et al 2015), casting doubt on the extent of this phenomenon. Meanwhile, estimates of the percent of hypertrophic chondrocyte-derived cells that become osteoblasts or osteocytes in limb cartilage range anywhere from 20% to 30% (Yang et al 2014a), 60% to 70% , or 80% (Yang et al 2014b), depending on the study.…”

Co-localization of Cell Lineage Markers and the Tomato Signal

“…By using the Rosa26 tdTomato lineage tracing system, our group and other investigators have shown that HCs can change their phenotype into bone cells during development [10][11][12][13][14] . Figure 1D).…”

The Mighty Chondrocyte

“…More recently, live cell imaging has been combined with lineage tracing to provide unprecedented real-time insights into progenitor cell origin and fate. In this issue, Jing et al (2015) use cell lineagetracing methods to reevaluate the current dogma concerning bone formation and reveal an unexpected fate for hypertrophic chondrocytes in mandibular condylar cartilage.It is well established that bone formation occurs by 2 distinct processes known as intramembranous and endochondral ossification. During intramembranous ossification (or dermal bone formation, as it is often called), mesenchymal cells differentiate directly into osteoblasts.…”

“…Recently, the 605457J DRXXX10.1177/0022034515605457Journal of Dental ResearchThe Mighty Chondrocyte research-article2015 application of CRE-LOX techniques for lineage tracing, in combination with analyses of gene function in mice, has convincingly shown in vivo that not all hypertrophic chondrocytes die; instead, they can transdifferentiate directly into osteoblasts (G. Zhou et al 2014;Park et al 2015). Jing et al (2015) provide the first in vivo evidence that mandibular chondrocytes directly transform into bone cells during development of the mandible. Using 1) triple transgenic mice containing collagen I (ColI) carrying a GFP reporter (green) to follow bone progenitors (osteoblasts); 2) an inducible CRE allele under the control of aggrecan or ColX to mark chondrocytes and hypertrophic chondrocytes, respectively; and 3) Rosa 26-tomato (red) to trace chondrocytes and hypertrophic chondrocyte lineages, Jing et al reveal that most osteoblasts in the growth plate derive from the chondroblast lineage.…”