The Extended Fertility of Naked Mole-Rats

Naked mole-rats exhibit few signs of aging across a life span. Only the queens bear young, but they can continue do so into old age. As this study notes, they achieve this feat via a number of mechanisms that ensure a continued supply of egg cells. This isn't just a matter of minimizing damage to these cells and their supporting tissues, but also generating new egg cells in adult life, unlike other mammals. It remains an open question as to whether there is any great realization yet to be found in this comparative biology that will benefit efforts to extend human fertility into later life. It doesn't hurt to look.

Unlike humans and other mammals, which become less fertile with age, naked mole-rats can reproduce throughout their remarkably long lifespans. For most mammals, including humans and mice, females are born with a finite number of egg cells, which are produced in utero via a process called oogenesis. Because this limited supply of egg cells depletes over time - some are released during ovulation, but most simply die - fertility declines with age. In contrast, naked mole-rat queens can breed right through old age, suggesting the rodents have special processes to preserve their ovarian reserve and avoid waning fertility. "There are three possibilities for how they do this: They are born with a lot of egg cells, not as many of these cells die, or they continue to create more egg cells after birth. My favorite hypothesis is that they use a cocktail of all three." Sure enough, researchers found evidence for each of the three processes.

The researchers compared ovaries from naked mole-rats and mice across different stages of development. Despite their similar sizes, mice live four years at most and start to show a drop in fertility by nine months, whereas naked mole-rats have a life expectancy of 30 years or more. They found that naked mole-rat females have exceptionally large numbers of egg cells compared to mice and that death rates of these cells were lower than in mice. For example, at 8 days old, a naked mole-rat female has on average 1.5 million egg cells, about 95 times more than mice of the same age. Most remarkably, the study found that oogenesis happens postnatally in naked mole-rats. Egg precursor cells were actively dividing in 3-month-old animals, and these precursors were found in 10-year-old animals, suggesting that oogenesis could continue throughout their lives.