Summary. Haemopoiesis is sustained by two main cellular components, the haematopoietic cells (HSCs) and the mesenchymal progenitor cells (MPCs). MPCs are multipotent and are the precursors for marrow stroma, bone, cartilage, muscle and connective tissues. Although the presence of HSCs in umbilical cord blood (UCB) is well known, that of MPCs has been not fully evaluated. In this study, we examined the ability of UCB harvests to generate in culture cells with characteristics of MPCs. Results showed that UCB-derived mononuclear cells, when set in culture, gave rise to adherent cells, which exhibited either an osteoclast-or a mesenchymal-like phenotype. Cells with the osteoclast phenotype were multinucleated, expressed TRAP activity and antigens CD45 and CD51/CD61. In turn, cells with the mesenchymal phenotype displayed a fibroblast-like morphology and expressed several MPC-related antigens (SH2, SH3, SH4, ASMA, MAB 1470, CD13, CD29 and CD49e). Our results suggest that preterm, as compared with term, cord blood is richer in mesenchymal progenitors, similar to haematopoietic progenitors.

Within the bone marrow stroma there exists a subset of nonhematopoietic cells referred to as mesenchymal stem or mesenchymal progenitor cells. These cells can be ex vivo expanded and Induced, either in vitro or in vivo, to terminally differentiate Into osteoblasts, chondrocytes, adlpocytes, tenocytes, myotubes, neural cells, and hematopoietic-supporting stroma. The multlpotential of these cells, their easy isolation and culture, as well as their high ex vivo expansive potential make these cells an attractive therapeutic tool. In this work we will review the information dealing with the biology of mesenchymal progenitors as it has been revealed mainly by ex vivo studies performed with bone marrow-derived cells. The discussed topics include, among others, characteristics of mesenchymal progenitors, evidence for the existence of a vast repertoire of un-Committed and committed progenitors both in the bone marrow and in mesenchymal tissues, a diagram for their proliferative hierarchy, and comments on mobilization, microenvironment, and clinical use of mesenchymal progenitors. Despite the enormous data available at molecular and cellular levels, it is evident that a number of fundamental questions still need to be resolved before mesenchymal progenitors can be used for safe and effective clinical applications in the context of both cell and gene therapies.

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.

Multipotent mesenchymal stromal cells (MSCs), often labeled mesenchymal stem cells, contribute to tissue regeneration in injured bone and cartilage, as well as in the infarcted heart, brain, and kidney. We hypothesize that MSCs might also contribute to pancreas and kidney regeneration in diabetic individuals. Therefore, in streptozotocin (STZ)-induced type 1 diabetes C57BL/6 mice, we tested whether a single intravenous dose of MSCs led to recovery of pancreatic and renal function and structure. When hyperglycemia, glycosuria, massive beta-pancreatic islets destruction, and mild albuminuria were evident (but still without renal histopathologic changes), mice were randomly separated in 2 groups: 1 received 0.5 x 10(6) MSCs that have been ex vivo expanded (and characterized according to their mesenchymal differentiation potential), and the other group received the vehicle. Within a week, only MSC-treated diabetic mice exhibited significant reduction in their blood glucose levels, reaching nearly euglycemic values a month later. Reversion of hyperglycemia and glycosuria remained for 2 months at least. An increase in morphologically normal beta-pancreatic islets was observed only in MSC-treated diabetic mice. Furthermore, in those animals albuminuria was reduced and glomeruli were histologically normal. On the other side, untreated diabetic mice presented glomerular hyalinosis and mesangial expansion. Thus, MSC administration resulted in beta-pancreatic islets regeneration and prevented renal damage in diabetic animals. Our preclinical results suggest bone marrow-derived MSC transplantation as a cell therapy strategy to treat type 1 diabetes and prevent diabetic nephropathy, its main complication.