Eye lens core proteins form around birth and stay

Researchers at the University of Copenhagen tested that suspicion with the same radiocarbon method archaeologists use to date ancient objects. Cold War nuclear tests had briefly doubled the carbon-14 in the atmosphere, leaving a dated signature in any protein built since.

The team measured carbon-14 levels in the lens crystallin proteins of people who had died at known ages. The proteins matched the atmospheric carbon-14 level from around each person's birth year rather than later in life, showing that the lens nucleus forms near birth and its core proteins are never replaced afterward.

Lens crystallins join dental enamel as one of the only tissues confirmed to last an entire lifetime without turnover, even though the lens itself keeps bending every time the eye focuses. The dating method was precise enough that the authors suggested it could help estimate age at death in forensic cases. The study measured lenses from only 13 people, taken at forensic autopsy, and did not examine how disease changes the pattern.

Born once, and never renewed again.