The art of building bone: emerging role of chondrocyte-to-osteoblast transdifferentiation in endochondral ossification

Abstract

There is a worldwide epidemic of skeletal diseases causing not only a public health issue but also accounting for a sizable portion of healthcare expenditures. The vertebrate skeleton is known to be formed by mesenchymal cells condensing into tissue elements (patterning phase) followed by their differentiation into cartilage (chondrocytes) or bone (osteoblasts) cells within the condensations. During the growth and remodeling phase, bone is formed directly via intramembranous ossification or through a cartilage to bone conversion via endochondral ossification routes. The canonical pathway of the endochondral bone formation process involves apoptosis of hypertrophic chondrocytes followed by vascular invasion that brings in osteoclast precursors to remove cartilage and osteoblast precursors to form bone. However, there is now an emerging role for chondrocyte-to-osteoblast transdifferentiation in the endochondral ossification process. Although the concept of "transdifferentiation" per se is not recent, new data using a variety of techniques to follow the fate of chondrocytes in different bones during embryonic and post-natal growth as well as during fracture repair in adults have identified three different models for chondrocyte-to-osteoblast transdifferentiation (direct transdifferentiation, dedifferentiation to redifferentiation, and chondrocyte to osteogenic precursor). This review focuses on the emerging models of chondrocyte-to-osteoblast transdifferentiation and their implications for the treatment of skeletal diseases as well as the possible signaling pathways that contribute to chondrocyte-to-osteoblast transdifferentiation processes.

Fig. 1

Existing paradigm for long bone endochondral ossification. a Primary endochondral ossification begins with the formation of a chondrocyte template during embryogenesis. Chondrocytes undergo hypertrophy beginning from the mid-diaphysis, eventually extending to the epiphyseal poles. Vasculature invades the forming bone, transporting marrow, mesenchymal stromal cells, and osteoclasts. Hypertrophic cells undergo apoptosis, aided by the removal of matrix by osteoclasts. Mesenchymal stromal cells differentiate into osteoblasts and then osteocytes. b Secondary ossification occurs at the epiphysis post-natally in rodents. Immature chondrocytes at the center of the epiphysis become hypertrophic to produce mineralized collagen and eventually undergo apoptosis. Vasculature invades and transports marrow, mesenchymal stromal cells, and osteoclast precursors. Bone formation initiates at the center and extends peripherally.

Fig. 2

Models of bone formation. a Endochondral ossification. Mesenchymal stromal cells develop into two different lineages, chondrogenic and osteogenic with no other intermediates. b Intramembranous ossification. Osteoblast development does not require the formation of a chondrocyte template. Mesenchymal stromal cells directly differentiate in an osteogenic lineage. c Chondrocyte to osteogenic precursor. Immature chondrocytes differentiate into an osteogenic precursor population which then differentiate into pre-osteoblasts and osteoblasts. d Dedifferentiation to redifferentiation. Hypertrophic chondrocytes dedifferentiate into immature chondrocytes, which directly differentiate to an osteogenic fate. e Direct transdifferentiation. Hypertrophic chondrocytes directly differentiate to osteoblasts.

Fig. 3

Lineage tracing of chondrocytes to osteoblasts. a The lineage trace construct consists of a tamoxifen-inducible CRE, which is driven by a promoter of a chondrogenic tissue, such as Sox9, Col2, Col10, or Acan. This in turn excises the stop codon blocking the ROSA-LSL-TdTomato construct. All further cells in the recombined lineage will express td-tomato. b, b′ ROSA-LSL-TdTomato floxed mice, positive for Col2-Cre^ERT2, were administered with tamoxifen at P3 and mice euthanized at P10. Coexpression of OSX and Tomato-red in cells of the primary spongiosa. c, c′ Coexpression of OSX and Tomato-red in cells of the epiphysis. d, d′ Coexpression of COL1 and Tomato-red in cells of the primary spongiosa. e, e′ Coexpression of COL1 and Tomato-red in cells of the epiphysis. COL1, collagen type 1; OSX, osterix; TOM, Tomato-red; DAPI, nuclear stain; PS, primary spongiosa; GP, growth plate; EP, epiphysis.

Fig. 4

Proposed model for thyroid hormone-mediated early post-natal development of the secondary ossification center (SOC). During embryonic and early post-natal development when thyroid hormone (TH) levels are low, epiphyseal chondrocytes express elevated levels of SHH, which acts through GLI1 to maintain these cells in proliferative immature state by activating Sox5/6/9 transcriptional activity. At P6/P7, rise in TH increases TRβ1expression and thereby IHH expression. IHH acts through GLI2 to decrease SOX9 and COL2 expression, while MMP13 and ADAMTS5 expression increases to deplete the COL2 matrix. Pre-hypertrophic chondrocyte (CC)s begin to express COL10 and OSX in the P8/P9 period. These in turn activate DMP1 and ALP in the SOC, meanwhile blood vessels invade from the periphery of the articular CCs. Text and image obtained from ref.