Fig. 8. A proposed model of MSC contribution to tissue maintenance. In this schematic representation of the transverse section of a simple vessel, MSCs lie in the basement membrane (red line), opposed to endothelial cells. Cues provided by the tissue-specific microenvironment coordinate a gradual transition (represented by green color gradient) from undifferentiated cells to progenitor and mature cell phenotypes. This process can occur naturally as represented by the dotted arrow (A). In case of tissue injury, undifferentiated MSCs can be mobilized directly into the tissue without the progenitor transition as represented by the curved arrow (B).