Tomorrow's skeleton staff: mesenchymal stem cells and the repair of bone and cartilage.

Stem cells are regenerating medicine. Advances in stem cell biology, and bone marrow-derived mesenchymal stem cells in particular, are demonstrating that many clinical options once thought to be science fiction may be attainable as fact. The extra- and intra-cellular signalling used by stem cells as they differentiate into lineages appropriate to their destination are becoming understood. Thus, the growth stimuli afforded by LIF, FGF-2 and HGF, as well as the complementary roles of Wnt and Dickkopf-1 in stem cell proliferation are evident. The ability to direct multi-lineage mesenchymal stem sell (MSC) potential towards an osteogenic phenotype by stimulation with Menin and Shh are important, as are the modulatory roles of Notch-1 and PPARgamma. Control of chondrocytic differentiation is effected by interplay of Brachyury, BMP-4 and TGFbeta3. Smads 1, 4 and 5 also play a role in these phenotypic expressions. The ability to culture MSC has led to their use in tissue repair, both as precursor and differentiated cell substitutes, and with successful animal models of bone and cartilage repair using MSC, their clinical use is accelerating. However, MSC also suppress some T-cell functions in transplanted hosts, and could facilitate tumour growth, so a cautious approach is needed.