(iii) The relevance of mesenchymal stem cells in vivo for future orthopaedic strategies aimed at fracture repair

The concept of a stem cell first emerged from studies on haematopoiesis where it was demonstrated that a rare clonogenic highly proliferative bone marrow (BM) cell, the haematopoietic stem cell (HSC), could give rise to all of the blood lineage cells. Hot on the heels of this work was the discovery of a second highly clonogenic and proliferative BM stem cell, later dubbed the mesenchymal stem cell (MSC) that was capable of giving rise to bone and cartilage and other stromal lineages. Whilst HSCs have been successfully utilized in the clinic for over two decades and have transformed the management of advanced haematological malignancy, the situation with MSCs is somewhat different. Despite the ability to generate billions of MSCs in vitro and over 300 proof of concept studies demonstrating in vivo fracture repair in animal models, MSCs have not yet been generally embraced by the orthopaedic community as the preferred agents to promote fracture repair.