Biology and clinical utilization of mesenchymal progenitor cells

Within the complex cellular arrangement found in the bone marrow stroma there exists a subset of nonhematopoietic cells referred to as mesenchymal progenitor cells (MPC). These cells can be expanded ex vivo and induced, either in vitro or in vivo, to terminally differentiate into at least seven types of cells: osteocytes, chondrocytes, adipocytes, tenocytes, myotubes, astrocytes and hematopoietic-supporting stroma. This broad multipotentiality, the feasibility to obtain MPC from bone marrow, cord and peripheral blood and their transplantability support the impact that the use of MPC will have in clinical settings. However, a number of fundamental questions about the cellular and molecular biology of MPC still need to be resolved before these cells can be used for safe and effective cell and gene therapies intended to replace, repair or enhance the physiological function of the mesenchymal and/or hematopoietic systems

Hydrocortisone regulates types I and III collagen gene expression and collagen synthesis in human marrow stromal cells

Hematopoiesis is the resultant of the orderly molecular and cellular interactions between progenitor cells and stroma. In vitro studies (Dexter-type cultures) have shown that initiation of hematopoiesis only occurs after establishment of a hydrocortisone-dependent layer of stromal cells. Although the molecular basis for the requirement of hydrocortisone are not well understood, data have shown that synthesis/assembly of extracellular matrix molecules (proteoglycans and fibronectin) is regulated by hydrocortisone. Since interstitial collagens are abundantly expressed in the marrow stroma, we investigated whether hydrocortisone may also modulate the expression of collagen types I and III. For these studies, human bone marrow fibroblast cultures were grown in standard culture medium either in the absence or presence of 10(-7) M hydrocortisone. Under both conditions, bone marrow fibroblasts synthesized collagen types I and III, and expressed the respective genes. However, hydrocortisone produced a decrease in the synthesis of interstitial collagens and also in the relative abundance of pro-alpha 1(I) and pro-alpha 1(III) mRNAs. The results of this study are consistent with the assumption that glucocorticoids regulate the expression of several extracellular matrix molecules in the marrow stroma and thus permit in vitro hematopoiesis to occur

The role of collagen in hematopoiesis

Several types of collagen, including types I, III, IV, V and VI, are produced by bone marrow stromal cells. Current information indicates that changes in collagen production result in profound alterations in the capacity of hematopoietic precursors to proliferate and differentiate. Although not definitively established, collagen molecules may be involved in the establishment and conformation of the stroma-associated extracellular matrix and/or in adhesive interactions with progenitor cells. The dynamic role of collagen in hematopoiesis is indicated by the observation that collagen production and processing are regulated by several factors such as glucocorticoids, cytokines, collagenases and collagenase-inhibitory proteins.