A good overview of the present consensus on stem cells and aging can be found at Science News Online: "over the past few years, researchers have found stem cells in many, perhaps most, of the body's organs and tissues. Even the brain, which scientists once thought never replaced its nerve cells during adulthood, is now known to have stem cells that make new nerve cells throughout life ... Imagine that, as a person ages, these fountains of cellular youth might start to run dry. As the supply of fresh cells dwindles, tissues would gradually decline and show signs of age. 'That was the initial mode' of how stem cells could be involved in aging ... Yet evidence is mounting that the connection between adult stem cells and aging is more complex. Some kinds of stem cell actually grow more abundant with age. And just as stem cells affect aging, the aging body affects stem cells. ... Whether the bodily declines that come with aging are due to the depletion of stem cells depends on which organ is in question - and on which scientist you ask. ... There's still a tremendous amount of debate about even the [blood stem cell] system, which is one of the best-studied systems."