Blood of world’s oldest woman hints at limits of life

“It’s estimated that we’re born with around 20,000 blood stem cells, and at any one time, around 1000 are simultaneously active to replenish blood,” says Holstege. During life, the number of active stem cells shrinks, she says, and their telomeres shorten to the point at which they die – a point called stem-cell exhaustion.
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https://www.newscientist.com/article/dn25458-blood-of-worlds-oldest-woman-hints-at-limits-of-life