Do Transplanted Hearts Assume the Age of Their New Host?

The miracles of modern transplant surgery can still sometimes boggle the mind. That the beating heart of one person can be sewn into the living body of another to save their life can seem like a Frankensteinian feat. But today, this kind of surgery is a standard and established medical procedure rather than an experimental one: In 2025, in the United States alone, more than 4,500 hearts were transplanted, according to the American Heart Association.

The tragedy is that it’s not available to everyone. Many thousands of people snake through long organ donation lists waiting their turn, and many of them die before their number comes up. That’s partly because everyone wants to receive a young, healthy heart rather than an old and weary one—the younger the better.

This could soon change, if recent findings about how organs age hold up. Read more: “If Only Technology Had a Heart” The desire for young organs has been partly driven by a speculative theory of rejuvenation: Research in mice known as heterochronic parabiosis, where an older and younger mouse’s circulatory systems are sewn together, suggested that the blood of a younger mouse could reverse age in an old one. Some hypothesized that something similar might happen with organ transplants.

But a team of scientists and surgeons from Harvard Medical School, together with a surgeon from Berlin, recently discovered that instead, an organ’s biological age can change to match that of its host after transplantation. When they sewed an old heart into a younger mouse or human, the organ seemed to turn back time, growing younger in its new dwelling, due to factors in the molecular environment. The reverse was also true, however: When a younger heart was surgically transplanted into an older mouse or person, the young heart aged to match the body of its recipient.

The organs “assimilate,” the scientists proposed, like a person who moves to a new place and learns the local language and customs so they can fit in. In mid September, the team published their paper as a preprint, meaning it has not yet been peer reviewed, so the work still needs to be evaluated and validated—but the findings could have implications both for transplant surgery and the science of aging and rejuvenation. I spoke with coauthor Jesse Poganik, whose lab at Harvard Medical School studies the biology of aging, about the future of transplant surgery, cell senescence, and epigenetic clocks.

Was this finding something you expected when you started out? Prior to this project, we were working with a system called heterochronic parabiosis, where you surgically join the circulatory systems of a young animal and an old animal to see how this affects tissue regeneration. It has been particularly popular in the aging field.