A developing human egg.
What's the News: Since the 1950s, it's been generally accepted that women are born with all the eggs they will ever have. One gets doled out with each menstrual cycle, and when they run out, you get menopause. But a smattering of papers over the last decade or so have indicated that that dogma might be incorrect: scientists found cells in the ovarian tissue of female mice that appear capable of producing new eggs. Now, working with donated tissue, researchers have found similar cells in human ovaries
. Headlines hyping the find have been spreading across the web, and we feel compelled to point out that this paper doesn't mean that we will be able to grow fresh new eggs in Petri dishes
, and it doesn't prove that in real, live women these cells actually mature into eggs that can develop into offspring
. It does, however, provide an interesting chance to see whether egg production by these cells can be jump-started using drugs. Everything You Need to Know About Eggs:
Scientists generally believe that the stem cells---cellular blank slates that can develop into more specialized cells---that will become eggs stop being produced in females before they are born. The proto-eggs that develop from those stem cells hang out in the deep-freeze of the ovaries until they are needed, at the time when a girl begins menstruating. Each menstrual cycle, many proto-eggs are taken out of the freezer and thawed, so to speak, but most will be tossed out: only one lucky egg will be cooked to perfection and released to wait for fertilization. So each cycle, the total count of eggs decreases, until the eggs run out and menopause begins. Estimates of how many eggs there are on average vary quite a lot, ranging from hundreds of thousands to millions; it's probably different for everyone.
What's the Backstory:
This team is led by Jonathan Tilly, who has been advancing the idea that eggs might be a renewable resource since 2004, when he published a paper saying that the math didn't work out for female mice: at the rate they used up their eggs, they should be running out long before they did. Poking around in mouse ovaries, Tilly noticed a few cells that looked a lot like those supposedly long-gone stem cells.
An interesting follow-up came in 2009, when Chinese researchers reported that they had found stem cells in mouse ovaries that appeared to make new eggs, and had confirmed that the cells did lead to eggs and thence to offspring. They gave the cells each a gene that made them glow green and injected them into females, and when the females had babies, some of them glowed green as well, proof that they came from eggs that developed from the original cells.
Mice aren't humans, though---can you imagine having 10 to 12 babies each time you got pregnant?---so this new paper, in which Tilly and co. report that they've found similar cells in human ovaries, is taking the hypothesis a big step forward.
What They Did:
Working with donated human tissue, the researchers extracted apparent stem cells, which were a tiny fraction of the total ovarian cells: about 1 in 10,000.
To see whether these cells behaved like egg-producing stem cells, they first give them a gene that made them glow green (for ease of identification) and then inserted them in a slab of human ovarian tissue that they implanted under the skin of a mouse, simulating the cells' natural environment.
After a couple of weeks, they found that the tissue had sprouted tiny globes, essentially like the proto-eggs mentioned above, that glowed green, indicating they had indeed come from the stem cells. It's not proof that the same thing happens in women, but it's promising.
Not So Fast:
This research is pretty exciting, but not for the reasons that some media outlets have been citing. It is unlikely that this finding will result in Petri dishes thronging with lab-grown eggs that women can then use to replenish their dwindling stores. Cells grown in the artificial environment of the lab for long periods of time tend to get a little strange: they accumulate mutations at a fast clip, which wouldn't bode well for pregnancies resulting from such eggs.
However, if it turns out that these cells do grow into fresh eggs in women, it suggests the possibility of new pharmaceutical treatments for infertility. Tilly has founded a biotech company to investigate whether certain chemicals can stimulate these cells and put them into overdrive, so that women whose existing supply has been damaged by chemotherapy, for instance, can take a pill to cause the growth of new eggs.
The Future Holds: Next up, Tilly and his collaborators will be investigating whether the proto-eggs they've grown from the stem cells develop into full eggs, at least in the lab, and whether those eggs can be fertilized. This could be a bit of a morally dicey proposition, but the experiments will likely take place in the UK, which has a regulatory structure set up to oversee research of this sort, ScienceNOW reports
. It's unlikely the team will ever get a chance to inject glowing green cells into women to see if they mature into full-blown eggs, but if they can gather enough indirect evidence in the lab that such a thing could be happening, they may be able to move forward with that. Reference: White, Y. A. R. et al. Nature Med. http://dx.doi.org/10.1038/nm.2669
Image courtesy of euthman / flickr