In humans, heart injury leads to the formation of scar tissue and cardiomyocyte hypertrophy, but the heart does not regenerate. In the December 13 Science, Kenneth Poss and colleagues, at Harvard Medical School, Boston, USA, show that zebrafish hearts are able to regenerate after injury without scarring (Science, 298:2188-2190, December 13, 2002).

Surgical removal of 20% of the ventricular myocardium from adult fish induces initial fibrin clot formation which is then replaced by cardiac myofibers and by two months after injury the hearts appeared grossly normal. Bromodeoxyuridine (BrdU) incorporation experiments showed extensive proliferation of cardiomyocytes near the amputation site. Cardiac injury in fish with a temperature-sensitive mutation in the mps1 gene — encoding a mitotic checkpoint kinase — led to the formation of large, connective-tissue scars in the absence of proliferation.

These results provide an interesting model to investigate the molecular mechanisms underlying cardiac regeneration and...

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