Human eggs appear to be protected against a certain type of age-related mutation. In a small study, researchers found no signs that mutations accumulate in the mitochondrial DNA of human egg cells as women get older, which may give us clues as to how they can stay fresh for decades.

“When we think about age-related mutations, we think about older people having more mutations than younger people,” says Kateryna Makova at Penn State University. “But expectation is not necessarily the truth.”

Mitochondria, which supply most of the energy to most of our body’s cells, are only passed down from mothers to their children. Although mutations in mitochondrial DNA are usually harmless, they can sometimes lead to complications, which particularly affect muscle and nerve cells given their high energy needs. “The oocyte [egg] provides this stockpile” says Ruth Lehmann at Massachusetts Institute of Technology, who wasn’t involved in the study.

Studies have shown that older mothers pass on more chromosomal mutations, prompting the widespread assumption that this also occurs among mutations to mitochondrial DNA. To study this, Makova and her colleagues used a DNA-sequencing method to identify any new mutations in 80 eggs collected from 22 women, aged 20 to 42.

They found that mitochondrial mutations in the women’s eggs actually didn’t increase as they aged. The same wasn’t true for the mitochondria in their salivary and blood cells. “I think that we evolved a mechanism to somehow lower our mutation burden, because we can reproduce later in life,” says Makova.

The researchers previously found that mutations in the mitochondrial DNA of macaque eggs increased until the animals were approximately 9 years old, their reproductive prime, then stayed constant. “It would be interesting to also look at younger women; this might be also the case in humans,” says team member Barbara Arbeithuber, also at Penn State University.