THE ROLE OF HUMAN MESENCHYMAL STEM CELLS IN REPAIR OF BONE

Abstract

Human multipotent stromal cells were first characterized by Alexander Friedenstein as fibroblastic colony-forming cells from the non- hematopoietic, culture vessel-adherent fraction of bone marrow1,2,3 . Bone marrow transplants had provided circumstantial evidence for the presence of precursor cells of the various components of bone marrow stroma: osteoblasts, adipocytes, chondrocytes, among others, while in vitro culture and differentiation of these colony-forming units provided more direct evidence of their existence. In vivo culture of the cells as implanted diffusion chambers or as ectopic transplants3 , showed formation of bony structures including bone marrow and established the CFU-F cells as a multipotent progenitor population capable of reconstituting bone marrow in an ablated recipient. These findings, along with their easy expansion in culture, initially suggested MSCs may have clinical applications in the areas of bone marrow transplant support and tissue repair. Since then, various groups have since studied the in vitro differentiation capacity of MSCs and the cells have also been extensively used as non-viral vectors for gene therapy. Early evidence of the role of MSCs in bone formation and homeostasis was provided by experiments where single colony-derived MSC cultures were implanted in host animals, resulting in an ectopic 2 ossicle containing bone cells, stroma, and adipocytes of donor origin and hematopoietic cells of recipient origin3 . Interestingly, not all of the implants form the complete ossicle. Some form only bone, some only form fatty deposits, and some only form fibrous tissue, incapable of supporting hematopoiesis. Less often, cartilaginous tissue is found. Because this heterogeneity implies that the original CFU-F population is composed of cells of different capacity or a range of developmental stages, considerable work has been done to find the a priori indicators of multipotentiality. One of the first markers for multipotency, STRO-1, was derived by injecting mice with CD34+ bone marrow cells, in the hopes of finding a monoclonal that would isolate the CFU-F population of bone marrow5,6 . While STRO-1 did isolate the CFU-F population, it also stained some B lymphocytes and nucleated red cells. Further work to get the required purity led to the development of a panel of markers, including CD29, CD34, CD44, CD106, and CD166. Unfortunately, due to variation in isolation, culture, and assay techniques, inconsistent results were obtained using the various panels and a means of isolating only multipotent cells has yet to be found7 . Because the gene expression and surface marker profile of even single-cell derived colonies changes over time in culture, full understanding of the cell state necessary and sufficient for multipotentiality, the “stemness” state, awaits techniques which can take a more comprehensive look at the gene expression and chromatin repression profiles of MSCs in culture.