Mesenchymal stem cells (MSCs), the archetypal multipotent progenitor cells derived in cultures of developed organs, are of unknown identity and native distribution. We have prospectively identified perivascular cells, principally pericytes, in multiple human organs including skeletal muscle, pancreas, adipose tissue, and placenta, on CD146, NG2, and PDGF-Rb expression and absence of hematopoietic, endothelial, and myogenic cell markers. Perivascular cells purified from skeletal muscle or nonmuscle tissues were myogenic in culture and in vivo. Irrespective of their tissue origin, long-term cultured perivascular cells retained myogenicity; exhibited at the clonal level osteogenic, chondrogenic, and adipogenic potentials; expressed MSC markers; and migrated in a culture model of chemotaxis. Expression of MSC markers was also detected at the surface of native, noncultured perivascular cells. Thus, blood vessel walls harbor a reserve of progenitor cells that may be integral to the origin of the elusive MSCs and other related adult stem cells.

The terms MSC and MSCs have become the preferred acronym to describe a cell and a cell population of multipotential stem/progenitor cells commonly referred to as mesenchymal stem cells, multipotential stromal cells, mesenchymal stromal cells, and mesenchymal progenitor cells. The MSCs can differentiate to important lineages under defined conditions in vitro and in limited situations after implantation in vivo. MSCs were isolated and described about 30 years ago and now there are over 55,000 publications on MSCs readily available. Here, we have focused on human MSCs whenever possible. The MSCs have broad anti-inflammatory and immune-modulatory properties. At present, these provide the greatest focus of human MSCs in clinical testing; however, the properties of cultured MSCs in vitro suggest they can have broader applications. The medical utility of MSCs continues to be investigated in over 950 clinical trials. There has been much progress in understanding MSCs over the years, and there is a strong foundation for future scientific research and clinical applications, but also some important questions remain to be answered. Developing further methods to understand and unlock MSC potential through intracellular and intercellular signaling, biomedical engineering, delivery methods and patient selection should all provide substantial advancements in the coming years and greater clinical opportunities. The expansive and growing field of MSC research is teaching us basic human cell biology as well as how to use this type of cell for cellular therapy in a variety of clinical settings, and while much promise is evident, careful new work is still needed.

We have identified a rare (0.05-0.1%) subset of human fetal bone marrow cells that contains muiltipotent hematopoietic precursors. The population of human precursor cells that express In bone marrow (BM), the main blood-forming organ in the developed mammal, cascades of stem-cell divisions give rise to most hematolymphoid cell populations (1-3). Of these, only totipotent hematopoietic stem cells (tHSCs) can reconstitute lethally irradiated animals by giving rise to all blood cells, including progeny HSCs.Isolation of candidate HSCs in the mouse required the development of assays for clonogenic precursors of the T [thyrnic colony-forming unit (CFU-T)], B, and myeloerythroid [splenic CFU (CFU-S)] lineages (4-6); such precursors lack detectable surface markers of the T, B, macrophage, granulocytic, and erythroid lineages [lineage-negative (Lin-)] (7) but express the Sca-1/Ly-6A antigen and low levels of the Thy-1 molecule (7-9). This Thy-11oLin-Sca-1+ population, representing -0.05% of mononucleated BM cells, is the only subset that initiates long-term BM stromal cultures; it is 1000-to 2000-fold enriched in the capacity to save lethally irradiated animals and reconstitute them long term with donorderived cells in all hematolymphoid lineages (7, 9). Independent attempts to isolate tHSCs have utilized other cellular properties (10-13) and demonstrated their activity in vitro or in vivo (14-16). In some studies, long-term reconstitution activity was separable from radioprotective and CFU-S activity (17). For example, the Thy-11OLin-Sca-l+Rh-123Io subset (comprising cells that take up little of the mitochondrial dye rhodamine 123) is enriched for self-renewal and longterm reconstitutive potential compared with its Rh-123 counterpart (18).Most attempts to identify human HSCs have utilized the high proliferative response of such cells in vitro in the presence ofhematopoietic cytokines and have revealed CD34 to be a potent cell surface marker of such progenitors (19,20); in mice, HSCs are not the only such cytokine-responsive cells (21). Several groups have developed human stromal cell-dependent long-term culture systems that have identified progenitors of the myeloerythroid type (22-24). The stromal culture system described here allows single human progenitor cells to differentiate into both the myeloerythroid and B-lymphoid lineages.Experimental in vivo hematopoietic assays are not practi- MATERIALS AND METHODSMonoclonal Antibodies. mAbs were purchased from Becton Dickinson (CD3, or from AMAC (CD35,. mAbs against HLA class I antigens were derived from hybridomas obtained from the American Type Culture Collection. mAbs to human CD34 (Tuk3) and to human Thy-1 (F15 421-5) were obtained from