Those unassuming, minnowsized, striped tropical fish swimming around in many household aquariums may hold the key to repairing cardiac muscle in humans hit by a heart attack.The common zebrafish, named for the five stripes on its side and known scientifically as Danio rerio, can regenerate organs and tissues. This ability, which some other fish and amphibians have but mammals do not, has spawned a school of investigators seeking to unlock...