Ageing is accompanied by a well-documented decline in the number and function of stem cells across multiple tissues, contributing to reduced repair capacity and increasing vulnerability. Therapies to reverse this are promising but have not yet been proven safe or effective in humans.
Yes, your stem cells do gradually run out as you age. Somatic stem cells -- the stem cells in your organs and tissues that are responsible for repair and renewal -- lose their functional capacity over time. Although stem cells are somewhat protected against ageing, they remain vulnerable to stress from within and from outside the body. This is one of the core processes driving physical ageing.
This happens in multiple tissues at the same time. In the brain, the production of new brain cells (neurogenesis) declines as neural stem cells decrease in number and activity. This is linked to cognitive decline and a greater risk of neurodegenerative diseases. In muscles, satellite cells -- the stem cells that repair muscles after damage -- become depleted: both their numbers and their function diminish. And in hair follicles, the activity of the associated stem cells decreases, contributing to age-related hair loss.
An additional factor is the condition of the mitochondria, the powerhouses of cells. A decline in the quality and function of mitochondria is associated with normal ageing as well as with falling stem cell activity. Whether the latter is a cause or a consequence has not yet been fully established. What is clear is that stem cell exhaustion contributes to age-related vulnerability, also known as frailty: a condition in which the body's physiological reserves shrink considerably.
Active research is under way into ways to slow or partially reverse this process. In old monkeys, infusions of genetically modified, ageing-resistant human stem cells led to fewer signs of ageing, better brain structure and cognition, and less inflammation, with no observed side effects in this 44-week experiment. In mice, age-related decline of immune stem cells was temporarily restored via mRNA therapy in the liver. Early clinical studies in humans with frailty suggest that stem cell transplantation appears to be safe, but the evidence for genuine effectiveness in humans is still insufficient.
An important caveat: all promising stem cell therapies are still in early stages of research. The results from animal studies cannot simply be translated to humans. Furthermore, stem cell transplantations carry a possible risk of tumour formation, which requires further monitoring and caution. There are currently no proven, safe therapies available that counteract stem cell exhaustion in healthy people.
Nine claims based on eight PMIDs. Strength of evidence for the phenomenon of stem cell exhaustion itself is strong (consistent findings across multiple tissue types). Strength of evidence for interventions is limited: animal models and early phase I/II studies in humans, with no large randomised trials.