Scientists claim to be a step closer to reversing the ageing process after rejuvenating worn out organs in elderly mice. The experimental treatment developed by researchers at the Dana-Farber Cancer Institute, Harvard Medical School, turned weak and feeble old mice into healthy animals by regenerating their aged bodies. The surprise recovery of the animals has raised hopes among scientists that it may be possible to achieve a similar feat in humans - or at least to slow down the ageing process. What we saw in these animals was not a slowing down or stabilisation of the ageing process. We saw a dramatic reversal - and that was unexpected," said Ronald DePinho, who led the study, which was published in the journal Nature. The Harvard group focused on a process called telomere shortening. Most cells in the body contain 23 pairs of chromosomes, which carry our DNA. At the ends of each chromosome is a protective cap called a telomere. Each time a cell divides, the telomeres are snipped shorter, until eventually they stop working and the cell dies or goes into a suspended state called "senescence". The process is behind much of the wear and tear associated with ageing. At Harvard, they bred genetically manipulated mice that lacked an enzyme called telomerase that stops telomeres getting shorter. When DePinho gave the mice injections to reactivate the enzyme, it repaired the damaged tissues and reversed the signs of ageing.