It's been a recurring theme in recent years that cell populations once thought to be static throughout much of life do in fact generate new cells at a slow rate or after injury. That such a process exists opens the door to efforts to speed it up as an alternative to other forms of regenerative medicine. For example, scientists have "shown the human body regenerates heart cells at a rate of about one percent a year, a discovery that could one day reduce the need for transplants. The study of 50 volunteers, using a dating method that detects traces of a carbon isotope left by Cold War nuclear bomb tests, raises the prospect of artificially stimulating the renewal process some day. ... Heart cells are unusual in that they stop dividing early in life. Doctors knew there were master cells called stem cells in the heart, but heart muscle usually simply forms scar tissue after damage and never fully regenerates. ... the rate at which the new cells are produced slows as we get older, with a young adult in their twenties renewing cells at a rate of about 1 percent a year, falling to half a percent a year by the age of 75. If you exchange cells at this rate it means that even if you live a very long life you will not have exchanged more than 50 percent of your cells. So at any given time your heart is a mosaic of cells you carry with you from birth and cells that that have been added later to replace cells that have been lost during life."