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As people age, so do all of our cells, which accumulate damage over time. But why our offspring don’t inherit those changes — effectively aging a child even before birth — has been a mystery. “When you are born, you don’t inherit your parents’ age,” says Yukiko Yamashita, a developmental biologist at MIT who studies the immortality of germline cells such as eggs or sperm. “For some reason, you are at zero.”

Experts once thought that germline cells might be ageless — somehow protected from the passage of time (SN: 3/10/04). But studies have shown signs of aging in eggs and sperm, dispelling that idea. So researchers have hypothesized that germline cells might instead reset their age after conception, reversing any damage. s

In a new study, scientists describe evidence that supports that rejuvenation hypothesis. Both mouse and human germline cells appear to reset their biological age in the early stages of an embryo’s development. A rejuvenation period that takes place after an embryo has attached to the uterus sets the growing embryo at its youngest biological age, dubbed “ground zero,” researchers report June 25 in Science Advances.

Understanding how germline cells reverse aging could help researchers develop treatments for age-related diseases, such as arthritis or Parkinson’s, says Vittorio Sebastiano, a developmental biologist at Stanford University School of Medicine who was not involved in the work. In such diseases, certain cells might become dysfunctional due to damage. Resetting the age of those cells could prevent them from causing problems.