Mammals are born with all the egg cells they will ever need, but how the cells stay alive and active for so long has been a mystery. Now, a pair of studies have revealed that it could all be down to the robustness of their proteins.

The two studies used traceable isotopes incorporated into growing mouse fetuses to measure the longevity of proteins in their ovaries. Many of these proteins survived much longer than proteins in the rest of the body. The presence of these “longevity” molecules and the support they provide to eggs and surrounding cells appear to be crucial for maintaining fertility.

The first study, by a team led by researchers at the Max Planck Institute for Multidisciplinary Sciences in Germany, analyzed oocytes in 8-week-old mice, when the animals were in their reproductive prime. About 10 percent of the oocyte proteins produced while the animals were in utero were still present.

The researchers then looked at older mice to see how long it took for these persistent proteins to break down. The answer: relatively speaking, not very fast at all. Some of the proteins remained in the mice’s ovaries throughout the animals’ short lives.

“Our data show that many proteins in oocytes and ovaries are unusually stable, with half-lives far exceeding those reported in other cell types and organs, including liver, heart, cartilage, muscle, and brain,” the researchers write in their published paper.

“The half-life of many proteins is much longer in the ovaries than in other organs. In addition, many other proteins have an exceptionally long lifespan in the ovaries.”

A second study by a team led by US researchers also found evidence of long-lasting ovarian proteins in young mice, including proteins that were present before the mice were born. Certain long-lasting proteins, such as ZP3, were identified for future study.

Some of these powerful proteins were present in the cell’s mitochondria, where a cell’s energy is generated. Because mitochondria are inherited as part of the egg cell from which a mammal grows, it could ensure that these crucial organelles can remain functional as they are passed from mother to offspring.

Eventually, even these much longer-lived proteins fade away and die, the researchers report. That could be linked to the natural decline in a woman’s ability to have children, the study suggests — and could ultimately point to ways to treat or at least better diagnose infertility.

The findings from these mouse studies still need to be reproduced in humans, but if they do, it would represent a major step forward in our understanding of fertility and how to keep eggs healthy.

“The exceptional persistence of these long-lived molecules suggests that they play a critical role in the lifelong maintenance and age-related decline of reproductive tissues,” the researchers behind the second study write in their published paper.

The research was published in Natural Cell Biology And e-life.