Researchers have taken an old mouse whose heart has thickened and enlarged with age, circulated a newly discovered protein in its blood, and saw that the heart reverted back to a more youthful state. They now believe the same effect can be achieved with elderly humans, using a new process that rejuvenates older muscle stem cell populations so they function like younger cells.

As the body grows older, stem cells within muscle tissues that are dedicated to the function of repairing damage become less capable of generating new muscle fibers. In older people, this translates to a diminished ability to regain strength and mobility after muscle has been injured.

The researchers from the Harvard Stem Cell Institute, led by Francesco S. Loffredo, turned to laboratory mice to study how their stem cells change with age. What the researchers have observed was that after they stitched together the circulatory systems of an old mouse and a young mouse - an old technique called parabiosis - the older mouse's heart was rejuvenated by the blood of the young mouse. They then searched for what factor was causing this rejuvenation, and found the protein called GDF-11. The details of the study have been published in the journal Cell.

In a different study conducted in 2011, Saul Villeda of the University of California, San Francisco was able to identify a factor in the blood that was causing the rejuvenating effect. His study was published in Nature Medicine.

Researchers in Stanford University, led by Helen Blau, director of Stanford's Baxter Laboratory for Stem Cell Biology, also studied mice, comparing stem cells and their behavior when affected by age. In their study, they saw that the changes were remarkable. Of the stem cells isolated from older mice, two-thirds were dysfunctional, and they remain dysfunctional even after being transplanted into younger muscles.

Through this method they were able to identify a defect inherent to aging muscle stem cells, and have found a way to overcome that defect. Their findings were published in the journal Nature Medicine.

The researchers feel hopeful about this new development. "We were able to show that transplantation of the old, treated muscle stem cell population repaired the damage and restored strength to injured muscles of old mice. Two months after transplantation, these muscles exhibited forces equivalent to young, uninjured muscles. This was the most encouraging finding of all," Benjamin Cosgrove, lead author of the study, told HealthDay.

"If we could isolate the stem cells from an elderly person, expose them in culture to the proper conditions to rejuvenate them and transfer them back into a site of muscle injury, we may be able to use the person's own cells to aid recovery from trauma or to prevent localized muscle atrophy and weakness due to broken bones," added Blau.

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