THE STROMAL CELLS GUIDE TO THE STEM-CELL UNIVERSE

Pluripotent hematopoietic stem cells have emerged as a heterogeneous p opulation of cells that differ in phenotype and repopulation kinetics. Stem cells in vitro and in vivo are dependent upon stromal cells for their proliferation and differentiation. Thus, stromal cells can be vi ewed as tools to analyze the physiological conditions that regulate st em cells. Stromal cell lines that support stem cells are infrequent, w hich supports the interpretation that stromal cells create distinct ni ches that regulate stem cell development, A model of stem cell mainten ance is presented that predicts that stromal cell-bound molecules prot ect stem cells from differentiation. The stroma compartment is highly adaptable and can change its function in response to external stimuli. Thus, it is tempting to speculate that the stroma acts as a translato r of peripheral signals for stem cells.