Cut a fifth of this fish's heart away, and it grows back completely in two months

Damage a zebrafish's heart and, instead of scarring like a human heart would, its remaining muscle cells start dividing again, rebuilding the lost tissue almost seamlessly within about two months. Researchers now study the fish to understand why that regenerative switch is permanently off in mammals — and whether it can be safely switched back on.

— Kenneth Poss et al., Heart regeneration in zebrafish — Science, 2002