as the sum of the cell–cell, cell–ECM, and cell–soluble factor interactions, in the context of the physical and geometric constraints that a cell may experience at a given time (Kaplan et al., 2007). The ability of the niche to determine the functional spectrum of stem cell activities leads to the hypothesis that stem cell niche microenvironments are critical in the definition of stem cell functions (Kaplan et al., 2007; LaBarge et al., 2007).
