Adult stem cells are of major importance to tissue function. They insure homeostasis, that is, proper balance between the death of cells and the life of those meant to replace them. But they only produce the types of cells present in the tissue they inhabit. In this way, stem cells present in the blood provide blood cells, those in the skin provide epidermal cells etc. In certain organs, such as the lungs or the liver, stem cells only step in in the event of very severe damage. They can then enable the full regeneration of the damaged tissue, or even of part of the organ. This regenerative capability of organs, which is very limited in humans, is much more developed in certain lower-order species, thanks to their sizable stock of stem cells (on occasion reaching 30% of the organism’s cellular mass).
We therefore have a pool of adult stem cells, spread out throughout the body but especially present in the blood, the marrow and abdominal fat; it is essential to our health and our longevity that this capital remain intact. Yet just like all other cells, stem cells endure constant attacks from free radicals, glycation, or inflammation. Their telomeres therefore also shorten, making the cell senile or forcing it to commit suicide. This is another major blow for the cell and its DNA which find themselves under attack from all sides. (Diagram: stem cell aging.)
Adult stem cells have three main characteristics: they multiply very quickly, they can transform into any cell type which is part of the embryological category to which they belong, and finally, they send signal molecules, called growth factors, which trigger regeneration processes within the cell. The danger inherent in the aging of stem cells is the same as for any somatic cell, but with one particularity: they will age faster if they are frequently used, as is the case for red blood cells. Indeed, rapid duplication leads to accelerated telomere shortening and senescence, or programmed cell death. We are therefore confronted with an entire mosaic of adult stem cells, whose cellular age will vary according to their location. However, we will see later on that it is now possible for us to reinvigorate our adult stem cell pool.