New evidence that adult stem cells are critical to human aging has recently been published on a study done on a super-centenarian woman that lived to be 115 years. At death, her circulating stem cell pool had declined to just two active stem cells from stem cell counts that are typically more than a thousand in younger adults. Super-centenarians have survived all the normal diseases that kill 99.9% of us before 100 years of age, so it has been a mystery as to what actually kills these hardy individuals. This recent data suggest that stem cell decline may be the main contributor to aging. If so, stabilizing stem cells may be the best thing one can do to slow your rate of aging.
There are many theories of aging that have been proposed. For example, damage to cells and tissues from oxidative stress has been one of the most popular fundamental theories of aging for more than half a century. Yet antioxidant substances or genes that code antioxidant enzymes have proven largely ineffective in slowing aging when tested in model animals. Thus, interest by scientists has shifted to other hypotheses that might provide a better explanation for the slow declines in function with age.
Stem cells provide one such promising mechanism of aging. Of course, we all know that babies are young and vigorous, independent of the age of their parents. This is because adults have embryonic stem cells that can generate young new cells needed to form a complete young baby. Indeed, these embryonic stem cells are the product of continuously evolving stem cell populations that go back to the beginning of life on earth over 3.5 billion years ago!
In adults, the mostly immortal embryonic stem cells give rise to mortal adult stem cells in all the tissues of the body. These adult stem cells can regenerate your cells and tissues as they wear out and need replacement. Unfortunate, adult stem cells also age, which leads to fewer cells and/or loss of function in cell replacement. As functional stem cells decline, skin and organs decline with age.
The somatic mutation burden in healthy white blood cells (WBCs) is not well known. Based on deep whole-genome sequencing, we estimate that approximately 450 somatic mutations accumulated in the nonrepetitive genome within the healthy blood compartment of a 115-yr-old woman. The detected mutations appear to have been harmless passenger mutations: They were enriched in noncoding, AT-rich regions that are not evolutionarily conserved, and they were depleted for genomic elements where mutations might have favorable or adverse effects on cellular fitness, such as regions with actively transcribed genes. The distribution of variant allele frequencies of these mutations suggests that the majority of the peripheral white blood cells were offspring of two related hematopoietic stem cell (HSC) clones. Moreover, telomere lengths of the WBCs were significantly shorter than telomere lengths from other tissues. Together, this suggests that the finite lifespan of HSCs, rather than somatic mutation effects, may lead to hematopoietic clonal evolution at extreme ages.