HThere’s a modern love story for you: A friend of mine recently asked me to donate my eggs. He and his wife, both professors at Harvard, wanted to have a child. But like many accomplished women with challenging careers, my friend’s wife was in her early 40s before she could seriously consider becoming pregnant, and it turned out she no longer had a healthy egg. As far as I’m concerned, this is about the most flattering thing someone can ask you: “Hey, buddy, would you mind giving us your gametes? We hope there is a small chance that our child will end up just like you.” I said yes.
There were hoops to jump through, including a fairly extensive health questionnaire, with information on all possible genetic problems that might run in my family. The IVF clinic had to check my egg reserve because it turned out that there is no date-specific switch that triggers menopause. On the contrary, our ovaries are slowly running out of eggs. We even start losing egg follicles—those small, fluid-filled sacs in the ovaries that hold our eggs until they develop properly—before we’re even born. If we have a congenital due date for the ovaries, it must be recorded in the uterus.
Call it the “empty basket” theory. While men continue to produce new sperm until they die, a woman is born with all the eggs she will ever have. Or rather: all egg sacs. Each month, as she goes through her ovulation cycle, the pituitary gland produces a quantity of follicle-stimulating hormone. In response, her ovaries begin to ‘ripen’ a handful of egg sacs. Normally only one of these will become a fully mature egg and make its way through the fallopian tube. It’s a kind of internal competition. Only the best survive.
This is probably what happened to my friend’s wife. Like almost every woman on the planet, she was born with about a million immature egg sacs. But since then, thousands of her follicles died and were reabsorbed into her body. By the time she became a teenager, she only had about 300,000 to 400,000 follicles left. From then on, she lost about a thousand every month. If she started ovulating at age 13, she was destined to run out of eggs sometime in her early 40s. That is exactly the point at which most women can no longer become pregnant without medical assistance.
Some women lose a few more eggs per month than average, and some women lose fewer. And for whatever reason, some women in their 30s and 40s retain more eggs of higher quality, while others seem to have more “bad” eggs: eggs with more chromosomal defects, eggs with defective mitochondria, or eggs that are simply, for whatever reason. reason, no longer up to the task. But we have no idea why our bodies evolved to throw away so many eggs in the first place.
I did worry that donating my eggs to my friends would jeopardize my own chances of having babies. Luckily not. Women who donate eggs do not appear to have any lower chance of becoming pregnant themselves. But no one could say whether donating eggs would cause me to enter menopause sooner than I would otherwise. (The data suggested I wouldn’t.) But why Doing we burn that many follicles every month? Why not lose a hundred instead of a thousand? How does the body know which eggs to store? Do good eggs become damaged over time, or are there only about 400 good follicles out of the million we are born with?
In other words, they are the most eggs a woman has duds? For almost half a century, the scientific community has thought that mammalian eggs might have an expiration date. That would at least go some way to explaining human menopause: perhaps it will help prevent genetic disorders. My friend’s body may have thrown away so many of her egg follicles before she reached her 40s because the eggs had major flaws in their genetic blueprints, such as more “double-strand breaks” in their DNA. There could be something wrong with the 1,000 eggs that most women throw away each month, probably due to the fact that eggs are so much harder to make than sperm, so there’s more opportunity for mistakes.
While half of your DNA came from your father and the other half from your mother, most of your mitochondria and cytoplasm came from your mother. Sperm is essentially an information transfer system that dumps the father’s DNA into the egg, while eggs must provide all the building materials to build that embryo. And that’s the main reason why eggs are much bigger than sperm: they’re not just half a set of blueprints; they are half a set of blueprints plus the entire factory.
Because sperm don’t need as much material, testicles don’t have to work as hard or as long to make their gametes. Ovaries, on the other hand, have to put in more effort, over a longer period of time, to mature an egg. Remember that the human fetus builds its egg sacs while still in the womb.
The longer a cell lives, the more likely it is to be damaged by the build-up of waste and free radicals. Mechanisms exist to repair damage, but these mechanisms become less reliable over time.
For the same reason, older women are more likely to have early miscarriages. So perhaps ancient humanoid bodies somehow anticipated these problems and threw away all those egg sacs to avoid giving birth to disabled babies. Because most mammals don’t live as long as we do, they may not have to deal with genetic damage from old eggs. There are some outliers, however, and they blow a hole in that theory. Elephants give birth at age 60, without any increase in genetic accidents. Some whales do that too.
Maybe we didn’t evolve to have menopause. Maybe it wasn’t selected for. Perhaps instead it was a natural side effect of our longer lifespans. Basically, bodies do just about anything they can to avoid death. So it’s not hard to imagine that evolution selects for traits that helped us avoid the grave. But in social species it can also be useful to have older people around. That could put further pressure on selecting genes that extend lifespan and lead to menopause in women.
The point of menopause is not that we stop ovulating. It’s that we keep it living past our predicted – and biologically tailored – expiration date. We have made it normal to grow old. That means that what’s interesting about menopause may not be menopause at all, but how people manage to avoid death.