A fish's solution to broken hearts. (Biology).

A zebrafish can regrow its heart within 2 months of having a significant portion of it surgically removed, according to a study in the Dec. 13, 2002 Science. "Zebrafish hearts can regenerate without scars," says Mark T. Keating, a Howard Hughes Medical Institute investigator at Children's Hospital in Boston, where he led the work.

This healing ability is rare, if not unprecedented, in vertebrates. Other researchers have found that some newts and salamanders have hearts that can heal when damaged, but the process seems to be scar formation, says Keating. Over the past decade, zebrafish have joined nematodes, fruit flies, and mice as experimental animals commonly and conveniently studied by biologists.

After the scientists cut out up to 20 percent of the zebrafish heart, they observed that blood cells quickly form a clot within the wound. Later, heart-muscle cells proliferate, and these eventually replace the excised portion. In a zebrafish with a genetic mutation inhibiting cell proliferation, the heart tissue fails to regemerate and a scar forms. That's similar to what happens to a human heart damaged by a heart attack or virus.

Keating suggests that regeneration and scarring are competing processes in damaged hearts. In zebrafish, regeneration wins out. In people, it doesn't. The goal, therefore, is to find therapies that tip the balance toward regeneration in the human heart.--J.T.