The Fountain of Youth: A Tale of Parabiosis, Stem Cells, and Rejuvenation

Transfusion (or drinking) of blood or of its components has been thought as a rejuvenation method since ancient times. Parabiosis, the procedure of joining two animals so that they share each others blood circulation, has revitalized the concept of blood as a putative drug. Since 2005, a number of papers have reported the anti-ageing effect of heterochronic parabiosis, which is joining an aged mouse to a young partner. The hallmark of aging is the decline of regenerative properties in most tissues, partially attributed to impaired function of stem and progenitor cells. In the parabiosis experiments, it was elegantly shown that factors derived from the young systemic environment are able to activate molecular signaling pathways in hepatic, muscle or neural stem cells of the old parabiont leading to increased tissue regeneration. Eventually, further studies have brought to identify some soluble factors in part responsible for these rejuvenating effects, including the chemokine CCL11, the growth differentiation factor 11, a member of the TGF-β superfamily, and oxytocin. The question about giving whole blood or specific factors in helping rejuvenation is open, as well as the mechanisms of action of these factors, deserving further studies to be translated into the life of (old) human beings.