Although chondrogenesis and osteogenesis are considered as two separate processes during endochondral bone formation after birth, recent studies have demonstrated the direct cell transformation from chondrocytes into bone cells in postnatal bone growth. Here we use cell lineage tracing and multiplein vivoapproaches to study the role ofBmpr1ain endochondrogenesis. Our data showed profound changes in skeletal shape, size and structure whenBmpr1awas deleted usingAggrecan-Cre^ERT2in early cartilage cells with a one-time tamoxifen injection. We observed the absence of lineage progression of chondrocyte-derived bone cells to form osteoblasts and osteocytes in metaphyses. Furthermore, we demonstrated the key contribution of growth plate chondrocytes and articular chondrocytes, not only for long bone growth, but also for bone remodeling. In contrast, deletingBmpr1ain early osteoblasts with3.6 Col 1-Crehad little impact on skeletal shape and size except for a sharp increase in osteoblasts and osteocytes, leading to a profound increase in bone volume. We conclude that chondrogenesis and osteogenesis are one continuous developmental and lineage-defined biological process, in whichBmpr1asignaling in chondrocytes is necessary for the formation of a pool or niche of osteoprogenitors that then contributes in a major way to overall bone formation and growth.

A recent breakthrough showing that direct trans-differentiation of chondrocytes into bone cells commonly occurs during endochondral bone formation in the growth plate, articular cartilage, and mandibular condylar cartilage suggests that chondrogenesis and osteogenesis are likely one continuous biological process instead of two separate processes. Yet, gene regulation of this cell transformation is largely unclear. Here, we employed cartilage-specific β-catenin loss-of-function (β-catenin ^fx/fx) and gain-of-function (β-catenin ^{fx(exon3)/ fx(exon3)}) models in the R26R^Tomato background (for better tracing the cell fate of chondrocytes) to study the role of β-catenin in cell trans-differentiation. Using histological, immunohistochemical, and radiological methods combined with cell lineage tracing techniques, we showed that deletion of β-catenin by either Acan-Cre^ERT2 or Col10a1-Cre resulted in greatly reduced cell trans-differentiation with a significant decrease in subchondral bone volume during mandibular condylar growth. Molecular studies demonstrated severe defects in cell proliferation and differentiation in both chondrocytes and bone cells. The gain of function studies (constitutive activation of β-catenin with Acan-Cre^ERT2 at ages of postnatal day 7, 4-weeks and 6-months) led to more bone cell trans-differentiation of chondrocytes in the mandibular condyle due to increased proliferation and accelerated chondrocyte differentiation with incipient osteogenic changes within the cartilage matrix, resulting in an increased volume of poorly-formed immature subchondral bone. These results support the notion that chondrogenesis and osteogenesis are one continuous process, in which β-catenin signaling plays an essential role in the cell trans-differentiation of chondrocytes into bone cells during mandibular condylar development and growth.

Objective: To detect myofibroblast formation on the tension side during orthodontic tooth movement in vivo and myofibroblast expression of alpha-smooth muscle actin (α-SMA) induced by tension both in vivo and in vitro. Materials and Methods: Fifty 6-week male rats were used in this in vivo study, and the right maxillary first molar was moved mesially, which served as the experimental group, and the left maxillary first molar served as the control. Rats were sacrificed at days 0, 3, 5, 7, and 14 after force loading. Myofibroblasts, identified with α-SMA, were examined through immunohistochemistry. For the in vitro study, human periodontal ligament (PDL) fibroblasts were obtained. Cyclic mechanical tension was applied to the fibroblasts for 0, 1, 3, 6, and 12 hours. Transmission electron microscopy was used to detect the ultrastructure of myofibroblasts. α-SMA mRNA gene expression was quantified by real-time quantitative PCR. The expression of α-SMA was detected by immunofluorescence and quantified by Western blotting. Results: In vivo, the myofibroblasts expressing α-SMA were identified both in the experimental group and in the control group. The expressions of α-SMA were increased in the tension areas of the experimental group over time, and reached the maximum in day 14. In vitro, fibronexus junctions and actin microfilaments in the cells could be found with transmission electron microscopy. Cyclic mechanical tension could significantly induce α-SMA expression at 12 hours (P < .01) than the controls. Conclusions: Myofibroblasts existed in the PDL. The expressions of α-SMA in the myofibroblasts were significantly up regulated under tension both in vivo and in vitro.

The vertebrate musculoskeletal system is known to be formed by mesenchymal stem cells condensing into tissue elements, which then differentiate into cartilage, bone, tendon/ligament, and muscle cells. These lineage-committed cells mature into end-stage differentiated cells, like hypertrophic chondrocytes and osteocytes, which are expected to expire and to be replaced by newly differentiated cells arising from the same lineage pathway. However, there is emerging evidence of the role of cell transdifferentiation in bone development and disease. Although the concept of cell transdifferentiation is not new, a breakthrough in cell lineage tracing allowed scientists to trace cell fates in vivo. Using this powerful tool, new theories have been established: (1) hypertrophic chondrocytes can transdifferentiate into bone cells during endochondral bone formation, fracture repair, and some bone diseases, and (2) tendon cells, beyond their conventional role in joint movement, directly participate in normal bone and cartilage formation, and ectopic ossification. The goal of this review is to obtain a better understanding of the key roles of cell transdifferentiation in skeletal development and diseases. We will first review the transdifferentiation of chondrocytes to bone cells during endochondral bone formation. Specifically, we will include the history of the debate on the fate of chondrocytes during bone formation, the key findings obtained in recent years on the critical factors and molecules that regulate this cell fate change, and the role of chondrocyte transdifferentiation in skeletal trauma and diseases. In addition, we will also summarize the latest discoveries on the novel roles of tendon cells and adipocytes on skeletal formation and diseases.