There is an intriguing amount of evidence to suggest that the stem cells remaining in the tissues of old people are still quite capable. If removed from the old cellular environments, many aspects of their behavior become similar to those of the same type of stem cell taken from a younger individual, at least in some reports. There is a greater level of accumulated cellular damage in old stem cells, but much of the evidence suggests that this does not provide as great a contribution to degenerative aging as do diminished numbers and diminished activity. Stem cell activity in the old is much declined from youthful levels, as I'm sure regular readers know. This activity is necessary for the support of tissues, supplying replacement somatic cells and generating signals that adjust cell behavior. The loss of regenerative capacity and consequent slow failure of tissue function an important part of the processes of aging.

As to whether the principal problem is loss of stem cells or that the stem cells are present but become perpetually quiescent, the evidence is varied. The situation is probably different for different stem cell populations, and to muddy the waters further, these are most likely overlapping issues. The stem cell populations react to the aging of tissues, meaning the rising level of damage and the changing signal environment that results from that damage. This reaction may be to self-renew less readily, decreasing the size of the stem cell population, or to remain quiescent and inactive for ever longer periods, decreasing the number of active stem cells at any point in time. Or both. The consensus theory on this process is that it is a part of the evolved balance between aging and cancer. As damage grows, so too does cancer risk, and stem cell decline can serve to reduce cancer risk at the cost of a slower decline into frailty and death.

Whether old stem cells are inherently dysfunctional is a question of considerable relevance to the practical development of stem cell therapies. The present direction in therapies is to use a patient's own cells, to take existing stem cells to generate more of the same for transplant, or to use those stem cells to create differentiated cells and tissues, again for transplantation. If aged stem cells are inherently dysfunctional, that would greatly limit the ability to use this class of therapies for older people, those who most need such treatments. But if, as seems to be the case, old stem cells are still capable in and of themselves, then this approach to regenerative medicine for age-related disease has a brighter future. Of course, the influence of the aged tissue environment still means that a challenging problem must be to solve to build effective regenerative therapies for the old: how to ensure that the fate of transplanted cells isn't just a repeat of what has already happened to the native cell populations? The regenerative medicine industry has to grapple with the causes and mechanisms of aging in one way or another, given that the vast majority of patients are in fact old, and the state of their aged tissues impacts cell therapy effectiveness.

Regenerative capacity of autologous stem cell transplantation in elderly: a report of biomedical outcomes