The majority of bones in the vertebrate skeleton develop by endochondral ossification, a process during which an intermediate cartilage template is successively replaced by bone. Many aspects of this process are relatively well understood; nevertheless, the origin of trabecular bone-forming osteoblasts and mesenchymal stem cells of the stroma has long remained under debate. Until recently, progenitors of these cell types were thought to enter the bone-forming structures from the periosteum together with the invading vasculature. Recent unexpected results revealed, however, that under physiological conditions differentiated hypertrophic chondrocytes give rise to both, osteoblasts and mesenchymal progenitor cells, thereby contributing to the formation of trabecular bone and bone marrow.
Keywords: Fracture healing; Ossification; Osteoblasts; Stem cells; Transdifferentiation.