The epithelial stem cell niche. The behavior of epithelial stem cells is regulated by external signals, provided by the microenvironment or niche in which stem cells reside. These signals include growth factors (for example Fgf10) and other stem cell regulatory factors secreted by the niche cells, which can be a wide variety of differentiated cell types, including fibroblasts, smooth muscle cells, endothelial cells, neurons as well as neighboring stem cell progeny (1). Another important component of the stem cell niche is the extracellular matrix (ECM), which acts as a reservoir for growth factors and provides mechanical cues to stem cells, which are translated into biochemical signals through integrins via a process called mechanotransduction (2). Finally, direct cell-cell contact between stem cells and their neighboring progeny, which is mediated by adherens and tight junctions, can also provide essential feedback information to their parent stem cells (3). Integration of these different types of niche signals regulates stem cell activity and behavior such as enhancing stem cell quiescence, promoting transient proliferation or differentiation, and maintaining stem cells in an undifferentiated state.