For more than 60 years, common knowledge — and Hollywood medical dramas — have led us to believe that women are born with all the eggs they'll ever have. Hell, this bit of female reproductive knowledge is a big plot point for sexy and fertility-challenged doctor Addison in that sexy doctors show Private Practice. But these scripts may be headed for major rewrites based on new research published this month in Nature Medicine.
That oft-quoted egg line is actually a Hollywood version of the scientific statement, “Women are born with fixed number of oocyte-containing follicles.” Oocytes aren't eggs (ovum), they're pre-eggs. These pre-eggs get their own proverbial baskets in the form of follicular cells that form protective, supportive layers about the oocytes.
These oocyte-packed follicules call the ovaries home. Pre-eggs and baskets undergo a maturation process called oogenesis that gives us mature eggs that are ready to escape their baskets via ovulation for possible fertilization.It's with pre-eggs (oocytes) that researchers have made a big breakthrough. They found egg-producing stem cells (oogonial stem cells or OSCs) in whole ovaries removed from sex-reassignment patients that were all young women of reproductive age.
That's a big damn deal.
Under the right conditions, stem cells are thought to develop into mature cells of various types. Those OSCs could develop into oocytes.
The proof's in the pudding, however, so researchers put these OSCs to the test. Researchers set-up a nice in vitro (e.g. culture dish) house, and OSCs made themselves at home by multiplying and undergoing oogenesis to produce oocytes. New pre-eggs!
To test OSCs out-of-a-dish, researchers performed a xenograft. That's when tissue or organs from one species is transplanted into or grafted onto an organism of another species. Our researchers had perfected their OSC-finding and oogenesis-provoking techniques on adult female mice (which have a reproductive system very similar to ours), so mice were the new human OSCs testing center. Bits of adult human ovarian tissue injected with OSCs were tucked under the skin of mice. After a week or two, these xenografts were analyzed and hot damn! There were immature follicles packed with oocytes.
So much for the idea that you can't make new eggs (or pre-eggs)!
This seemingly radical idea of oocyte generation in adults isn't brand new. As stated earlier, these researchers had been working with adult female mice and first published that work in 2004. It would be an understatement to say this earlier mice work, with parallels drawn human adult females, was met with skepticism.
“We were reminded that humans are not big mice, only if you're in Disneyland, because it was a mouse study,” remembers one of the researchers, Dr. Johnathan Tilly in this Nature video. “Obviously we had to work very hard to try to prove this in humans.”
Tilly and colleagues have certainly made huge progress in advancing the idea that adult female humans have the ability to produce new pre-eggs, but…. can it be done naturally in the human female body? That's an even bigger test. What does all this mean for in vitro fertilization(IVF)? Way too early to say.
Can we extend the fertility time window? Should we extend the fertility time window? Those questions deal with a whole hell of a lot more than being able to make pre-eggs under experimental conditions.
There is a lot to be said and debated about before we rewrite the book on human female reproduction. But based on this work, we better start sharpening our pencils.