Biologists in the US today publish what they say is the first clear evidence linking the ageing of individual cells to the ageing of the whole body.

They found that the number of senescent cells in the skin of baboons increases exponentially as the animals grow older.

Over time, cells losetheir ability to divide and enter a state of "replicative senescence". Humancells typically replicate between 60 and 90 times before senescence sets in.

John Sedivy of Brown University in Rhode Island, the research leader, says the phenomenon has been known for 40 years but "whether it promotes the ageing of our bodies is highly controversial. While it may make intuitive sense, sceptics say 'Show us the evidence'. The first solid evidence is in this study".

The Brown scientists counted the number of cells with biochemical markers of senescence in small skin samples taken from the arms of baboons aged from five to 30 - roughly equivalent to a human age range from 15 to 90 - living in a research colony. The results, published in the journal Science, show that the number of senescent cells increased exponentially with age to reach 20 per cent in the oldest animals.

The researchers plannext to assess senescencein baboons' muscle and blood cells.www.brown.edu