Young mammals are capable of feats of regeneration: even in humans, it has been known for young children to regenerate lost fingertips. That capacity fades with age, however. Researchers are investigating the biochemistry of this behavior for much the same reasons as they look at regenerating species such as salamanders - if the capacity is there, perhaps it can be restored in adults. "Researchers, working with mice, found that a portion of the heart removed during the first week after birth grew back wholly and correctly - as if nothing had happened. ... This is an important step in our search for a cure for heart disease, the No. 1 killer in the developed world. We found that the heart of newborn mammals can fix itself; it just forgets how as it gets older. The challenge now is to find a way to remind the adult heart how to fix itself again. ... Previous research has demonstrated that the lower organisms, like some fish and amphibians, that can regrow fins and tails, can also regrow portions of their hearts after injury. ... In contrast, the hearts of adult mammals lack the ability to regrow lost or damaged tissue, and as a result, when the heart is injured, for example after a heart attack, it gets weaker, which eventually leads to heart failure. ... The researchers found that within three weeks of removing 15 percent of the newborn mouse heart, the heart was able to completely grow back the lost tissue, and as a result looked and functioned just like a normal heart. The researchers believe that uninjured beating heart cells, called cardiomyocytes, are a major source of the new cells. They stop beating long enough to divide and provide the heart with fresh cardiomyocytes."