There are things women hate to hear most during Chuseok. "When are you getting married, Hanni?" "Don't you have plans for children?" Even as times change, these questions rarely disappear. Just because a woman is older, the term "advanced maternal age" gets attached, and she worries that if there is a problem with the baby later, it might be her fault.
Scientists are easing women's worries. An egg does not age as much as a woman's actual age. In other words, the egg's clock runs more slowly. If the secret of the egg's "slow aging" is revealed, it could also help infertility treatments.
◇ Eggs have fewer mutations than other cells
In mammals, including humans, females are born with oocytes that will later become eggs. Unlike other human cells, oocytes have a long lifespan. They can remain healthy for more than 40 years. While the lifespan of an ovulated egg is only 12 to 24 hours, immature eggs live much longer. Scientists are uncovering, one by one, the secrets that allow oocytes to remain healthy for a long time.
On Aug. 8, a research team led by Professor Kateryna Makova and Dr. Barbara Arbeithuber at the University of Pennsylvania reported in the international journal Science Advances that mitochondrial DNA (mtDNA) in women's oocytes accumulates few mutations with age.
Most genetic material, DNA, is in the cell nucleus, but about 0.1% is also in mitochondria, the energy-producing organelles in the cytoplasm. A baby inherits mitochondrial DNA from the mother's egg. If mutations occur there, genetic diseases can develop that severely damage the brain, muscles, or heart.
The researchers collected and compared 80 immature eggs along with blood, oral cells, and saliva from 22 women aged 20 to 42. For this, they used a specialized analytical method called "ultra-accurate duplex sequencing." By reading and comparing both strands of DNA, the method almost completely filters out errors that can occur during analysis. Thanks to this, they were able to observe changes in the egg's DNA with fine precision.
The results were surprising. In blood, oral cells, and saliva, genetic mutations increased markedly with age, but not in eggs. In eggs with many mutations, the changes appeared mainly in regions without particular functions, rather than in DNA regions responsible for key functions.
The team interpreted this as the result of "purifying selection." Purifying selection is nature's self-defense mechanism, in which organisms weed out harmful mutations and preserve essential functions. Put simply, human eggs are designed so that, even as time passes, DNA responsible for key functions is protected, and only small changes occur in less important regions.
Makova said, "People usually think mutations increase with age, but expectations do not always align with truth," adding, "It appears that humans evolved mechanisms by which immature eggs protect themselves so reproduction is possible even at later ages."
◇ Slowing waste processing to prevent reactive oxygen species
There is another secret to the oocyte's slow aging. In Jul., a research team led by Dr. Elvan Böke at the Centre for Genomic Regulation (CRG) in Barcelona, Spain, reported in the international journal The EMBO Journal that "immature eggs deliberately slow their own waste-cleaning speed to minimize cellular damage."
Lysosomes and proteasomes break down and recycle waste proteins in cells. They are essentially "garbage processing plants." However, the cleanup process requires energy, and harmful byproducts called reactive oxygen species are produced, which can damage DNA and cell membranes. Immature eggs intentionally slowed the cleanup speed to prevent such damage.
The researchers analyzed more than 100 immature eggs obtained from 21 women aged 19 to 34. Activity of lysosomes, proteasomes, and mitochondria in eggs was at about 50% of surrounding cells, and it slowed further as cells matured. Just before ovulation, they also observed eggs expelling lysosomes outside the cell in a "major cleanup."
In a 2022 study published in Nature, Böke's team also found that immature eggs deliberately skip certain metabolic processes to suppress the generation of reactive oxygen species. Taken together, the three studies suggest that human eggs protect core DNA from mutations and defend themselves through strategies of energy conservation and cleanup-speed control.
For reference, Böke received the EMBO Medal last year, awarded to researchers under 40 who have made outstanding contributions to the life sciences in Europe. She was recognized for pioneering research into the physiological mechanisms that allow oocytes, which are immature eggs, to remain healthy in a dormant state for decades.
When women face those dreaded questions every holiday, remember the discoveries of these two women scientists. Even as time passes, your body is far stronger and wiser than you think.
References
Science Advances (2025), DOI: https://doi.org/10.1126/sciadv.adw4954
The EMBO Journal (2025), DOI: https://doi.org/10.1038/s44318-025-00493-2
Nature (2022), DOI: https://doi.org/10.1038/s41586-022-04979-5