Science used to say you were born with a set number of eggs. That’s it. Game over. Once you ran out, usually around menopause, the bank account hit zero. For seventy years this was the gospel. Biology was simple. Finite supply. Declining reserves. The end.
Then researchers started asking questions.
Over the last two decades we’ve found something odd in adult ovaries. In mice. In cows. Pigs. Primates. And yes in humans. Cells that look like they might be making new egg cells called oocytes. Are they viable? Nobody knows for sure yet. But the mere possibility has ignited a war in reproductive biology. The old dogma says ovaries are static warehouses. The new idea suggests they’re dynamic factories. Maybe even fixable.
Jonathan Tilly from Northeastern University puts it bluntly. “What if that wasn’t the case?” he asks. “What if the fixed number idea is wrong?”
The Ghost of Zuckerman
The idea that ovaries regenerate isn’t new. It was around in the 1920S. It just got squashed. Hard.
Enter Solly Zuckerman. A British scientist in the mid-1900S who counted eggs in animals. He didn’t see any new ones forming. So he concluded the process was dead on arrival. His work was so authoritative that for half a century no one argued with it. Why would you? It was settled science.
Then in 2004 Tilly and his team did the math.
They looked at a process called follicular atresia — basically when eggs die off before they mature. The numbers didn’t add up. The rate of death was too fast given the number of eggs remaining in older women. Tilly called it out. If eggs die that quickly but still exist later in life then something must be replenishing them. He proposed the existence of oogonial stem cells (OSCs). Stem cells that sit in the ovary like sperms sit in testes ready to go.
It sounded crazy. Then in 2009 scientists in Shanghai isolated these cells in adult mice. By 2012 they found them in humans too.
Glow-in-the-dark Proof
By 2017 things got weirder.
Tilly’s team genetically engineered female mice so any newly formed oocyte glowed fluorescent green under a microscope. They lit up. Bright green. Not only did the mice keep making new eggs but those eggs could actually produce viable offspring. It was direct evidence. Or so it seemed.
Fast forward to 2023. The team reported a snag. While these OSCs hang around into old age in mice they seem to shut down. Key developmental genes turn off. It’s the same pattern in human tissue. It might explain why menopause happens. The factory workers are still there. But they’ve gone on strike.
This doesn’t stop the skeptics. Some still worship at the altar of Zuckerman.
Evelyn Telfer from the University of Edinburgh knows the pain. She chairs reproductive biology there and knows that scientific consensus is a stubborn beast. “It’s difficult to get funding” she says. You fight the community you’re part of. She was wary of Tilly at first. Cautious even.
After working with human ovarian tissue herself her view shifted. She sees cells transforming into oocyte-like structures. Forming follicles. Those fluid-filled sacs where eggs grow. “These cells underwent a transformation” she says. She still has doubts about whether they make viable human eggs. But she’s sold on the existence of the cells themselves.
Not everyone is buying it.
Aaron Hsueh from Stanford is fierce in his disagreement. He hates the antibodies Tilly uses to prove these cells exist. He calls them suspect. Tilly fires back. Says the antibodies are commercial standard. Accuses Hsueh of cherry-picking data and ignoring nearly 100 papers that back up his findings.
Science isn’t pretty when it fights. It’s messy. Abrupt.
“You’re part of a community… if you’re going against those views it’s difficult to get funding.”
— Evelyn Telfer
The Broken House
So what does this mean for us?
Tilly sees a future. Treatments for infertility. Saving ovarian function in cancer patients. Maybe even delaying menopause or treating its symptoms. He wants to extend the healthspan. But there’s a catch. The ovary is complex. It’s not just eggs floating in vacuum.
Think of it like a neighborhood.
Telfer points out that eggs need support. They sit in follicles. They need hormone-producing cells. As we age that whole ecosystem rots. Tissues get fibrotic. Signals break. Tilly likes a different analogy. The stem cells are people. The ovary is the house.
Post-menopause the house is in disrepair. The structure fails. Even if you have builders inside the walls (the stem cells) they can’t work. The environment doesn’t support them. The plumbing leaks. The roof caves in.
Tilly isn’t giving up on the house though. His lab is trying to build new ones outside the body. They’re making organoids. Little lab-grown pieces of ovarian tissue. These contain the patient’s own OSCS and hormone support cells.
The goal isn’t necessarily babies. At least not right now.
It’s about hormones.
Imagine storing a woman’s stem cells. Maybe before menopause. Maybe after. Then growing them in a dish. Implanting that tissue elsewhere in the body like in an arm. The tissue makes hormones naturally. No side effects. No ineffective replacement therapy. Just a biological backup generator.
Too Soon?
They’ve isolated “dormant” OSCs from women decades post-menopause. Not published yet but the work is happening. Tilly says it changes his perspective entirely. Why do ovaries fail? Maybe they don’t fail. Maybe they just go dormant. Can you wake them up?
It sounds promising. Maybe too promising.
We are years away from therapy. Decades even. We don’t know if those organoid implants are safe. If they work long term. “We need to do a lot of basic work” Telfer warns. It’s not a light switch you flip. It’s a tangled web of biology we barely understand.
If dormant regenerative cells do persist in us all they change everything. But if they don’t we go back to the bank account.
Zero balance.
