5. Heterochronic Parabiosis Suggests that Systemic Milieu Can Modulate Biological Age

Heterochronic parabiosis can test whether systemic factors can shift aged tissues toward more youthful function. Animal models suggest that it provides partial rejuvenation (57, 58). In deleteriome terms, this approach may change both the rate of accumulation and the overall burden of damage (e.g., by providing access to younger organs). The limitations are ethical considerations, complexity, and safety. It is unlikely to be a scalable human therapy, but parabiosing with young artificial systems, when technology makes it possible, is a possibility. Most importantly, this approach requires sustained exposure to a youthful systemic milieu. Heterochronic parabiosis affords the old parabiont continuous access to the organs of the young parabiont and differs from approaches such as young blood transfusions and removal of aged components from the blood (e.g., via plasmapheresis), whose efficacy is questionable.