The reason that an otherwise healthy person grows old and dies remains a mystery. Scientists have suggested several suspects for why people's bodies wear out with age, including accumulated damage to DNA, free radicals, and the shortening of telomeres?the caps on the ends of chromosomes. While each of these factors may play a part, biologists acknowledge that their understanding of aging is incomplete.
http://www.sciencenews.org/articles/20080209/bob8.aspEnter stem cells. Scientists have long known that people have small reservoirs of stem cells in some of their tissues, such as bone marrow. These stem cells are distinct from those found in newly fertilized embryos?the more controversial embryonic stem cells. The embryonic type can become any type of cell in the body.
Adult stem cells, in contrast, can normally generate new cells only for the tissue in which they're found: blood cells for blood, intestinal cells for the intestines. As old cells in these tissues are damaged or wear out, nearby stem cells can manufacture new ones to take their place. At the same time, the stem cells produce more copies of themselves, maintaining a seemingly indefinite pool of cells capable of churning out a stream of replacement cells.
