The word "parabiosis" doesn't seem so strange: para means "beside" and bios means "life." But in some of the scientific fields, it means something fascinating and occasionally creepy.
In physiology, parabiosis means that two living creatures are joined together. Conjoined twins have spontaneous parabiosis. But, it can also refer to a more unsettling practice—when researchers surgically join two animals together.
They do this to study, for example, the effects of hormones as they flow through artificially wedded circulatory systems. One hundred and fifty years of such studies have helped advance our understanding of endocrinology, immunity and oncology. And when two animals, one young and one old, are joined, they can reveal surprising things about aging.
For Nature, Megan Scudellari writes:
By joining the circulatory system of an old mouse to that of a young mouse, scientists have produced some remarkable results. In the heart, brain, muscles and almost every other tissue examined, the blood of young mice seems to bring new life to aging organs, making old mice stronger, smarter and healthier. It even makes their fur shinier. Now these labs have begun to identify the components of young blood that are responsible for these changes.
One of the researchers involved in such work, Tony Wyss-Coray of Stanford University, says, "We are restarting the aging clock." But others are more circumspect—Amy Wagers of Harvard University says that what is happening is more akin to restoring function, not de-aging. Instead of prolonging life, the work might help older people recover from illness or heal from surgery.
Except in work from the University of California, the older partner from joined young-old rat pairs did live four to five months longer than controls. This work and that of others has lead researchers to think that there are certain compounds circulating in blood that coordinate aging in tissues throughout the body. Some of these factors have been identified: Oxytocin, a hormone important for social bonds, declines as people age and can help regenerate muscles by activating muscle stem cells, for example.
And if you’re worried about the animals in these experiments, Thomas Rando, a Stanford neurologist, explains that the procedures are carefully thought out and not undertaken lightly. Mice to be joined first get a chance to know each other, are given careful, anaesthesized surgery, monitored closely and can be separated.
More recently, some researchers have tried to bring these findings to humans. In September, a start-up formed by Wyss-Coray started testing the use of young people’s plasma—the liquid, yellow component of blood that holds blood cells—to address Alzheimer’s disease. (The results aren’t in yet.) But experiments in mice showed that the older animal in young-old pairs did have increased neuron growth. The young mice had decreased growth because of their exposure to old blood.
One relevant concern is that activating stem cells might not be entirely desirable. “My suspicion is that chronic treatments with anything—plasma, drugs—that rejuvenate cells in old animals is going to lead to an increase in cancer,” Rando told Nature. “Even if we learn how to make cells young, it's something we'll want to do judiciously.”
If young blood’s anti-aging effects do pan out, people in the future may take the specific active factors that researchers have identified rather than vampirish blood transfusions. That would reduce the idea’s strangeness and make it just another innovative medicine.