THE ROLE OF OSTEOCHONDRAL PROGENITOR CELLS IN FRACTURE REPAIR

The repair of a fracture necessarily entails synthesis of osseous tiss ue requiring the transformation of undifferentiated osteochondral prog enitor cells to mature osteoblasts and chondrocytes, Owen and Friedens tein proposed that there are stem cells for all mesenchymal tissues, r esident in bone marrow throughout life, that have a lineage comparable to that described for hematopoiesis. Subsequent with this initial stu dy, marrow derived and periosteal derived progenitor cells have been s hown to produce bone and cartilage in numerous in vivo and in vitro st udies. The differentiation process appears to depend heavily on the in fluences of numerous cytokines, especially the transforming growth fac tor beta superfamily, Initial cartilage formation from progenitor cell s is important in any secondary fracture repair. In the in vitro study of chondrogenesis, the marrow derived progenitor cells were shown to differentiate into their terminal phenotype, the hypertrophic chondroc yte, as indicated by the detection of Type X collagen messenger ribonu cleic acid and protein. A concomitant elevation in the alkaline phosph atase level suggests that these cells are ready to mineralize. Despite the importance of these cells in fracture repair, the characterizatio n of these cells and the mechanism of their differentiation have only begun to be explored.