Researchers continue to delve into the details of regeneration in lower animals: "Bony fish like the tiny zebrafish have a remarkable ability that mammals can only dream of: if you lop off a chunk of their heart they swim sluggishly for a few days but within a month appear perfectly normal. How they accomplish this - or, more importantly, why we can't - is one of the significant questions in regenerative medicine today. .. [Researchers have] identified a fish heart cell population that is the source of this astonishing healing feat ... well established cardiomyocytes remaining after injury had likely regressed to a more 'youthful' state, started dividing again to replenish lost cells, and then matured a second time into new heart muscle. The group also showed cardiomyocytes recaptured lost youth in part by re-activating the production of proteins associated with cell proliferation, factors typically expressed in immature progenitors. Human hearts cannot undergo these types of regenerative changes on their own. When damaged by heart attack, our heart muscle is replaced by scar tissue incapable of contracting. However, prior to heart failure, damaged mammalian heart muscle cells enter a save-yourself state known as 'hibernation,' in which they cease contracting in an effort to survive. ... During heart regeneration in the zebrafish we found that cardiomyocytes displayed structural changes similar to those observed in hibernating cardiomyocytes. Because of these similarities, we hypothesize that hibernating mammalian cardiomyocytes may represent cells that are attempting to proliferate."