Scientists have successfully reversed aging in rats' hearts - paving the way to a treatment for humans. 

Researchers at Cedars-Sinai Heart Institute injected fresh cardiac cells from newborn lab rats into old rats.

Previously, this experimental method has only been used as a way to repair damage after a heart attack.

But in this study, the American team - who performed the world's first cardiac stem cell infusion in 2009 - have demonstrated it can also reverse aging.

Experts say the breakthrough could revolutionize medicine. In the study, 22-month-old rats - who were considered old - received stem cells from four-month-old rats. 

Across the board, all of them experienced improved heart function, improved their exercise capacity by an average of 20 percent, and regrew hair faster than rats that didn't receive the cells.

They also demonstrated longer heart cell telomeres - compound structures located at the ends of chromosomes that shrink with age. 'Our previous lab studies and human clinical trials have shown promise in treating heart failure using cardiac stem cell infusions,' said lead author Dr Eduardo Marbán, director of the Cedars-Sinai Heart Institute. 

'Now we find that these specialized stem cells could turn out to reverse problems associated with aging of the heart.' 

Stem cells are a basic type of cell that can change into another type of more specialized cell through a process known as differentiation.

Similar to a fresh ball of clay, they can be shaped and morphed into any cell in the body.

They grow in embryos as embryonic stem cells, used to help the rapidly growing baby form the millions of different cell types it needs to grow before birth.