Apart from a gradual loss in the number of stem cells, there is a problem with the remaining stem cells. During much of our life, we have a healthy diversity of cells that have acquired different mutations, making us a mosaic of genomes. As we age, our stem cells acquire mutations, some of which cause them to proliferate more rapidly. These rapidly multiplying stem cells are not necessarily the best for regenerating tissues, but because they have a growth advantage, they outcompete their counterparts. Consequently, old age leaves us with stem cells that have all descended from just a few
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