Via EurekAlert, more details on the role of Wnt signaling in the regeneration of complex organs in zebrafish: regeneration "involves creating cells that can take any number of new roles. This can be done by re-programming cells that already have a given function or by activating resident stem cells ... a particular kind of cell-to-cell communication, called Wnt/Beta-catenin signaling, regulates the fate of these as-yet undeveloped cells as an embryo forms. ... These streams of signals also tell stem cells in adult organisms what functions to undertake. Once tissue formation starts, something has to tell it to stop before growth gets out of hand. ... We show that Wnt/Beta-catenin signaling is activated in the regenerating zebrafish tail and is required for the formation and subsequent proliferation of the progenitor cells of the blastema ... It is most likely the inability of humans to form a blastema in the first place that renders us unable to re-grow arms and legs ... the same genes for turning on and turning off growth and development are found in humans, and drugs exist that can regulate this pathway."