Human bone marrow mesenchymal stem cells in vivo.

Great confusion still exists amongst cell biologists, musculoskeletal and other specialists interested in regenerative medicine regarding the in vivo identity of human bone marrow (BM) mesenchymal stem cells (MSCs). Contrary to views held in some quarters, methods for the robust identification and purification of BM MSCs are now well established. Human BM MSCs represent a phenotypically homogeneous cell population that share an identical phenotype with marrow adventitial reticular cells (ARCs), which are stromal cells similar in nature to pericytes. When an extensive panel of markers is used to characterize BM MSCs, it appears that the diverse MSC markers described in different laboratories are expressed on the same cell population. Rare cell phenotypical analysis and in vitro colony forming unit-fibroblast (CFU-F) assays produce no compelling evidence that BM MSCs circulate in healthy man. Furthermore, although investigators speak of a number of specific MSC markers, a true marker of MSC 'stemness' and multipotentiality has not yet been defined since culture-expanded MSCs may lose some of these markers, but remain multipotential. This knowledge provides a platform for understanding MSCs in vivo leading to novel approaches for therapy development, including in situ tissue engineering.