Enlarge Image Expert repair. Muscle cells (green) in a zebrafish heart regenerate lost tissue 7 days (left), 14 days (middle), and 30 days (right) after an injury. Credit: Adapted from Jopling et al., Nature, 464 (25 March 2010) Zebrafish hearts can take a licking and keep on ticking. Even if they lose up to 20% of a ventricle, the animals form a clot that stops bleeding within seconds and then gradually replace the lost tissue. Within a month or so, they are back to normal. The impressive repair work is accomplished not by stem cells in the heart but...