A number of researchers seek to bring the regenerative prowess of some lower animals into the realm of human medicine. Zebra fish are one candidate, if we can just understand how they do it. Scientific American reports that zebra fish "regenerate heart muscle in two synchronized steps. Within five days of removing about 20 percent of an adult zebra fish heart ventricle, undifferentiated progenitor cells begin to line the injured area and turn into cardiac muscle cells, which grow and divide, building new heart muscle. Meanwhile, developmental genes in the epicardium - a cell layer that surrounds the entire heart and influences embryonic heart development - switch on, activating the epicardial cells. Most of these cells form a new layer to cover the wound and the regenerating heart muscle, but some also create blood vessels for the growing muscle ... A growth factor signal orchestrates the two processes so that they converge to mend the zebra fish heart ... If the communication between the two cell types is blocked, the zebra fish heart starts to scar and cannot completely regenerate."