The Incredible (Renewable?) Egg

It’s hard to keep up anymore; things are changing so quickly in fertility medicine. Just in the last week, a Harvard researcher discovered stem cells in the human ovary and a 66 year old just became a mom to twins in Switzerland. There goes the rug, being pulled out from under our scientific feet yet again.

Testicular Stem Cells

A couple of years ago, we published a paper confirming the presence of cells in adult men that were long though to exist: testicular stem cells. These are “multipotent” cells unlike most other cells in the body: they not only renew themselves, but they can also become sperm and may even be coaxed into becoming other organs in the body. The really convenient thing is that they are not derived from embryonic stem cells (a political minefield) a fact that makes them prime candidates for stem cell-based treatments in the future.

The Dogma About Eggs and Ovaries

But eggs are not sperm. Sperm are made constantly throughout a man’s life, like blood cells, and therefore it makes sense that there are stem cells running the show. However, for at least as long as I have been in medicine, it has been thought that women are born with all of the eggs they will ever have, some 1-2 million, and lose them throughout life, most obviously through monthly ovulation, until they are gone. The basket empties of eggs somewhere in the 40’s for most women, about 10 years before menopause. This lack of new egg production has led to the belief that, unlike within the testicle, adult stem cells do not exist in the ovary.

Ovarian Stem Cells

But Dr. Jonathan Tilly at Harvard thinks otherwise. He believes that women indeed have adult stem cells in the ovary that might be able produce new eggs during a woman’s lifetime. And maybe even after her traditional reproductive window has closed.
Dr. Tilly first showed this in mice and published it in 2004. He recently extended this work to women by locating an incredibly scare biological resource: fresh ovaries donated by Japanese women undergoing sex change procedures. Within this tissue, he found cells that showed classic stem cell characteristics using genetic markers. But this only means that they “look” like stem cells. To prove that they can “act” like real stem cells, he isolated these cells in a Petri dish and watched them turn into real, live, immature eggs. More convincingly, he then transplanted them into mice and they continued to develop into even more mature eggs, passing through meiosis in some cases which is a characteristic process occurring only in eggs and sperm.
So what are the implications of this technologically mind-bending work? Hard to know, but down the line it may mean that women may be able to make new eggs when we thought that they couldn’t.  Maybe their baskets never really empty of eggs and that their fertility can be extended beyond what we now think is possible.
Regarding the 66 year old women who just had twins…it’s not clear to me how many genetic parents were involved, how long it took, and how heroic the effort was, but I can guarantee you that she will have her hands way too full for the next several years to keep up with this science.