Every type of tissue is supported by its own dedicated stem cell population, delivering a supply of daughter somatic cells that replace losses and maintain tissue function. Unfortunately, stem cell function declines with age. This has numerous causes, all of which descend from the underlying accumulation of molecular damage outlined in the SENS research proposals for rejuvenation biotechnologies. Downstream of those causes, stem cells become less active due to some combination of internal damage, damage to their niche of supporting cells, and changes in the signaling environment. The latter two classes of issue appear more influential in the best studied stem cell populations, such as the satellite cells of muscle tissue.

Thus most research and development intended to restore stem cell function is presently focused on trying to override signaling or cell programming in order to force stem cells into greater activity, regardless of the present state of their environment. This may produce benefits to tissue function that are sizable enough to be worth the effort and cost of development, but one cannot just forever ignore the underlying damage of aging with impunity.

For one thing, not every aspect of aging can be fixed by throwing cells at the problem: there are protein aggregates and other forms of molecular waste that are not adequately cleared for reasons that have little to do with stem cells. There is mitochondrial dysfunction throughout cell populations. And so forth. Further, is most likely that stem cells falter in function in response to a damaged environment because this acts to limit cancer risk, though at the cost of a drawn out decline. Thus many of the strategies outlined in this open access paper may turn out to have increased incidence of cancer as a side-effect, even when they achieve meaningful gains in the near term.

However, we can balance that expectation against the animal studies of telomerase gene therapies to lengthen telomeres. In mice that approach both improves stem cell function and reduces cancer risk. In that case, it may be that improved function of the immune system in anti-cancer immunosurveillance offsets the raised risk due to forcing damaged cells into greater activity. That said, a great deal more work is required to understand where the line is drawn on cancer risk in the many approaches to improved stem cell function.

Rejuvenating Strategies of Tissue-specific Stem Cells for Healthy Aging