The native cellular microenvironment of stem cells is termed the stem cell niche, of which there are three primary features: cell secreted factors, cell-cell interaction, and the extracellular matrix (Watt et al. 2000). Cell-secreted factors regulate many stem cell activities.

For example, TGF-beta and Wnts, two families of common secretedfactors found across many species, are responsible for proliferation via asymmetric divisions. At least two types of TGF-beta were also found to be important in differentiation of neural crest stem cells.

. Although these factors are important, direct cell-to-cell contact is also necessary for other local signals that control stem cell activity. Examples of this are the two transmembrane proteins Notch and Delta. In Drosophila, the progeny of the sensory organ precursor is regulated by Notch activity (Watt et al. 2000).