Mark T. Keating and his colleagues at Harvard Medical School wounded the hearts of adult zebrafishes by surgically removing 20 percent of the muscle from the lower chamber. Most of the creatures survived the surgery and resumed swimming. Within a week, the scientists found, the altered fish could match their normal counterparts in speed and agility. In addition, after two months the injured ventricles had returned to their standard size and shape--with no evidence of scarring--and were contracting normally. The team theorizes that zebrafish produce new heart-muscle cells, or cardiomyocytes, in response to injury, which allows them to regenerate undamaged heart muscle. Humans, in contrast, experience scarring after the heart muscle is harmed. Keating notes that "the implication, which isn't proven, but is quite exciting, is that by enhancing human cardiomyocyte proliferation after heart injury, one may be able to enhance cardiac regeneration and reduce cardiac scarring."