text: Phenotypical and functional properties of human bone marrow mesenchymal progenitor cells.
  Bone marrow stroma provides the microenvironment for hematopoiesis and is also the source of mesenchymal progenitors (mesenchymal or marrow stromal cells [MSC]) that may serve as long-lasting precursors for bone, cartilage, lung, and muscle. While several studies have indicated the differentiation potential of MSC, few studies have been performed on the cells themselves. In an attempt to further expand our knowledge on these cells, we have performed studies on their cell cycle, immuno- and adhesive-phenotype, ex vivo expansion, and differentiation properties. MSC cultures have been initiated from human bone marrow low-density mononuclear cells and maintained in the absence of differentiation stimuli and hematopoietic cells. The homogenous layer of adherent cells thus formed exhibits a typical fibroblastlike morphology, a population doubling time of 33 h, a large expansive potential, and cell cycle characteristics including a subset (20%) of quiescent cells. The antigenic phenotype of MSC is not unique, borrowing features of mesenchymal, endothelial, and epithelial cells. Together, MSC express several adhesion-related antigens, like the integrin subunits alpha4, alpha5, beta1, integrins alphavbeta3 and alphavbeta5, ICAM-1, and CD44H. MSC produce and functionally adhere to extracellular matrix molecules. When incubated under proper stimuli, MSC differentiate into osteoblasts or adipocytes. Taken together, these results demonstrate that adherent marrow-derived cells cultured in the absence of hematopoietic cells and differentiation stimulus give rise to a population of cells with phenotypical and functional features of mesenchymal progenitors. The existence of a subset of quiescent cells in MSC cultures seems to be extremely significant, since their number and properties should be enough to sustain a steady supply of cells that upon proliferation and commitment may serve as precursors for a number of nonhematopoietic tissues.