NEWS AND VIEWS Microarraying the cellular microenvironment

The in vivo stem cell microenvironment or niche is composed of an intricate blend of extracellular matrix proteins, soluble protein factors, immobilized protein factors, proteoglycans, small molecule signals, in some cases mineralized tissue, and numerous adjacent cell types, all of which likely vary in space and time. In concert, these many components present the cells with an ‘array ’ of biochemical and biomechanical signals, such that the cell is continually faced with the complex tasks of sensing these inputs, processing the signals through complex signal transduction and gene regulation networks, and executing cell behavioral or fate choices. This native microenvironment therefore poses a daunting challenge to the fields of stem cell biology and engineering: howdoes one analyze the relative importance of the individual signals of this medley in regulating cell function? In addition t