Stem cell niche is a phrase loosely used in the scientific community to describe the microenvironment in which stem cells are found, which interacts with stem cells to regulate stem cell fate. The word 'niche' can be in reference to the in vivo or in vitro stem cell microenvironment. During embryonic development, various niche factors act on embryonic stem cells to alter gene expression, and induce their proliferation or differentiation for the development of the fetus. Within the human body, stem cell niches maintain adult stem cells in a quiescent state, but after tissue injury, the surrounding micro-environment actively signals to stem cells to either promote self renewal or differentiation to form new tissues. Several factors are important to regulate stem cell characteristics within the niche: cell-cell interactions between stem cells, as well as interactions between stem cells and neighbouring differentiated cells, interactions between stem cells and adhesion molecules, extracellular matrix components, the oxygen tension, growth factors, cytokines, and physiochemical nature of the environment including the pH, ionic strength (e.g. Ca2+ concentration) and metabolites, like ATP, are also important. The stem cells and niche may induce each other during development and reciprocally signal to maintain each other during adulthood.
http://en.wikipedia.org/wiki/Stem_cell_niche