Unlike humans and other mammals, who become less fertile with age, hairless rats can reproduce throughout their long lives. A new study, published in Nature Communications, sheds light on the unique processes that give these rodents what appears to be “eternal” fertility.
“Naked mole rats — or shaved mole rats — are the strangest of mammals,” says Miguel Brieño-Enríquez, lead author and researcher in the Department of Obstetrics, Gynecology, and Reproductive Sciences at the University of Pittsburgh. “They are the rodents that live the longest. . , almost never suffer from cancer, do not feel pain like other mammals, live in underground colonies and only the queen can have offspring.
But, in his opinion, “the most amazing thing is that they never stop reproducing: they don’t suffer from a decrease in fertility as they age and we want to understand how they do it”.
In most mammals, including humans and mice, females are born with a finite number of eggs, which are produced in utero by a process called oogenesis. As this limited number is depleted over time – some are released during ovulation, but most simply die – fertility declines with age.
Shaved mouse queens can reproduce to a very old age, suggesting that these rodents have special processes to preserve their ovarian reserve and prevent fertility decline.
On the other hand, queens of shaved mice can reproduce to a very old age, suggesting that these rodents have special processes to preserve their ovarian reserve and prevent fertility decline.
“There are three possibilities: they are born with many eggs, they die less or they continue to create more eggs after they hatch”, explains Brieño-Enríquez. “My preferred hypothesis is that they use a cocktail of all three.” Indeed, Brieño-Enríquez and her team found evidence of each of the three processes.
The researchers compared ovaries from rodents and female mice at different stages of development. Despite their similar size, mice live a maximum of four years and begin to show decreased fertility at nine months, while naked mole rats have a life expectancy of 30 years or more.
An abnormally high number of eggs
They found that female rats have an exceptionally high number of eggs, compared to mice, and that the death rates of these cells were lower than in mice. For example, at eight days old, a female naked mole rat has an average of 1.5 million eggs, about 95 times more than mice of the same age.
And what’s more surprising, the study found that oogenesis occurs after birth in these animals. Egg precursors were actively dividing in 3-month-old mice, and these cell precursors were found in 10-year-old mice, suggesting that oogenesis could continue throughout life.
“This discovery is extraordinary”, says Ned Place, co-author of the article and professor at the School of Veterinary Medicine at Cornell University (New York). “This challenges the dogma established nearly 70 years ago that female mammals are endowed with a finite number of eggs before or shortly after birth, with no addition to the ovarian pool afterwards.”
When the queen dies or is removed from the colony, subordinate females compete to take her place and become reproductively active. Any woman can become queen
Miguel Brieño-Enríquez, lead author (University of Pittsburgh))
Only the dominant female can reproduce.
Shaved rats live in colonies that can range from several dozen to hundreds of individuals. Like bees or ants, community members share the tasks of defense, digging tunnels, caring for their young, and gathering food. Only the dominant female in the colony can reproduce, and she suppresses the reproduction of others to maintain her status.
“Unlike bees or ants, a female mousetrap is not born a queen”, explains Brieño-Enríquez. “When the queen dies or is removed from the colony, subordinate females compete to take her place and become reproductively active. Any woman can become queen.”
To better understand this process, the researchers took three-year-old females from the colony to trigger their reproductive activation and compared these new queens with subordinate females. They found that the non-reproductive subordinates had egg precursor cells in their ovaries, but the cells began to divide only after transitioning to the queen.
We hope to use what we’re learning from mice to protect ovarian function later in life, prolong fertility and prevent disease.
Miguel Brieno-Enriquez
“This is important because, if we could discover how they do it, we could develop new drug targets or techniques to help human health”, says the Spanish researcher. “Although humans are living longer and longer, menopause continues to occur at the same age. We hope to use what we’re learning from mice to protect ovarian function later in life and prolong fertility.”
Furthermore, he adds, “the ovary is more than just a potential baby factory. Ovarian health influences the risk of cancer, heart disease and even life expectancy. Therefore, a greater knowledge of this area can help us to find ways to improve health in general”, he concludes.
Reference:
M. Brieño-Enríquez, N. Place et al. “Postnatal oogenesis leads to an exceptionally large ovarian reserve in naked mole rats.” Nature Communications (February 2023).